Enmity build up on untargetable bosses

[Eisenhower]

[...] Healers are the only exception here and there's no good reason why it should be a thing.
[...] why would they be granted extra disadvantages?

First of all, I do not consider paying normal costs for performing role actions a disadvantage. Healers are not in any way an exception as they continue to perform their role - healing - despite the boss being removed. That ASTs can perform similarly to a WHM but have a a greater difficulty to manage enmity is a problem inherent to AST design, not encounter design.

This contradicts how DPS driven this game is [...]

The quote you highlighted has nothing to do with the DPS focus the game has at the moment, or healer DPS. I simply mentioned buffered enmity as a resource to manage. It is the choice of the player as a tank to determine how much enmity to generate for each specific encounter. The tank should generate enough enmity to keep the boss throughout the encounter, not only from the DPS roles but also the healer. If a tank foregoes enmity to deal more damage ("pushing the limits", "being a good player" - whatever one would want to call it) and subsequently gets the healer killed despite commendable effort on the healer's part to avoid this, it is the tank that has failed to balance the aspects of the tank role.

It's not much of a "test" if the result would be instant death upon the boss's return [...]

No it would not. That would be just terrible. I have never stated that "Sephiroth Dives" are desired.

But currently (as far as I can remember at least) only one encounter has this element. Just as it is possible that a future encounter may include such an element again, it is equally if not more likely that such an element won't be present considering what we've come across in the game so far. I personally can't advocate change based on such conjecture. The picture you paint regarding healers getting destroyed the moment a boss returns to fight is in my opinion misleading.

[Lyrica_Ashtine]

Snip

I believe I've mentioned several times how it wasn't a problem with the old content with how White Mages work and how it isn't a problem now with how the content is. Please read more carefully. I've brought this to attention as it may present a much bigger issue for future content and/or future jobs. Even if it's not a serious problem now, it's more of a pain in the ass gimmick more than anything for everyone:
Healers take more damage than tanks, losing GCDs left and right till the tank re-establishes hate. Which can be one hit or several hits depending on the situation
Tanks lose output potential as they're forced to perform enmity combos over damage combos which they normally could without that transition
DPS lose positionals because the boss is spinning left and right

First of all, I do not consider paying normal costs for performing role actions a disadvantage. Healers are not in any way an exception as they continue to perform their role - healing - despite the boss being removed. That ASTs can perform similarly to a WHM but have a a greater difficulty to manage enmity is a problem inherent to AST design, not encounter design.

If not the healers being at a disadvantage, then tanks are at a disadvantage. If you consider "performing role actions" as "normal", then tanks performing enmity actions on the boss would be normal too. But they are unable to do so as the boss cannot be targeted or is invulnerable.

The quote you highlighted has nothing to do with the DPS focus the game has at the moment, or healer DPS.

I brought it up as it's something healers would do aside from just spamming that single button to keep the tank's HP bar up. Enmity buffer would only last for so many actions and someone else in the topic shared his/her experience in thordan EX. Said person also mentioned he/she was DPSing. That's one way to artificially lower your enmity for a phase. But why would good healers who can do both mending as well as maiming be punished for pushing their limits?

It's not much of a choice for a certain tank job either. Paladins pretty much have to repeat their Rage of Halone combo to build up enough enmity to last through the Ultimate End phase.

No it would not. That would be just terrible. I have never stated that "Sephiroth Dives" are desired.

But currently (as far as I can remember at least) only one encounter has this element. Just as it is possible that a future encounter may include such an element again, it is equally if not more likely that such an element won't be present considering what we've come across in the game so far. I personally can't advocate change based on such conjecture. The picture you paint regarding healers getting destroyed the moment a boss returns to fight is in my opinion misleading.

Although not as extreme as T9: https://youtu.be/4v4nmg-fsIs?t=14m15s

While there was time to Provoke, any form of action being taken before the next tank action has effect will outhate the effect of Provoke.
Then there's the thing that Square-Enix will have to consciously design content that doesn't open up with a hard hitting move. I believe there would be more coordination in preparing for the next phase by positioning properly. Rather than take a beating and try to recover.

I'll just repeat it once more, as you seem to keep ignoring this:
It wasn't a problem back then due to how Shroud works and still isn't a problem for White Mages
It isn't a serious problem now as current content doesn't open up a phase with heavy damage moves
It may be a problem later unless content is specifically designed to:
...have nothing that will utterly destroy someone upon phase change
...have short invulnerable/untargetable phases
...have sufficient amount of time to build enmity for the phase where they cannot for the duration of it
...make any tank/healer combination work (even if sub-optimal) if the duration of such phase is long

This issue doesn't only concern healers (except for White Mages, I guess) and there's no valid reason why this is a thing. If "management" and "ability" was really a thing, consider it from a tank perspective:
Certain adds spawn with initial enmity build on adds:
Oppressor and Oppressor 0.5
Living Limb appears with enmity build up on Living Liquid
Ser Janlenoux and Ser Adelphel appear with enmity already build up on King Thordan
Everyone builds up enmity on The Manipulator while attacking it's legs (or healing for healers)

If you consider A4(S), why can't this be a thing for other encounters?

[Eisenhower]

To preface, if you're going to quote, please consider actually including text other than "Snip" so I know what you're referring to.

[...][1] [Healers dying, tanks failing to be the main target, dps losing dps]
[...][2] tanks performing enmity actions on the boss would be normal too. But they are unable to do so as the boss cannot be targeted or is invulnerable.
[...][3] [Healers dealing damage, Thordan EX anecdote]
[...][4] Paladins pretty much have to repeat their Rage of Halone combo to build up enough enmity to last through the Ultimate End phase.
[...][5] [T13 video] While there was time to Provoke, any form of action being taken before the next tank action has effect will outhate the effect of Provoke.
[...][6] I'll just repeat it once more, as you seem to keep ignoring this:
[...][7] [Adds, bosses with pre-set enmity on spawn]

(1) Excuse my paraphrasing, but the described scenario appears to be easily solved by not panicking. The healers aren't killed outright and should move to the tank position. The tank is performing their role as usual and the DPS roles are losing a most likely trivial amount of damage.

(2) When the boss is displaced, the tank's role against the boss is effectively suspended. They cannot manipulate values against the boss in a meaningful way. Thus tanks should have ensured that enmity is sufficiently buffered, whilst considering their DPS and mitigation responsibilities. Healers on the other hand continue to function normally, their responsibilities and the costs for their actions should remain as normal.

(3) I will never argue against healers dealing damage. What is important to highlight is that in the anecdote the healer became the main target but wasn't killed outright after the phase change despite several strikes from the boss. This is in no way a healer being "punished for pushing their limits" as the tank got the boss back without deaths. If this happened every time and each time the healer died without neither the healing nor tank being able to do anything about it, then yes, it would be "punishing". Instead it shows that a tank can solve the enmity problem with their available tools and reacting on the spot.

(4) Paladins having trash enmity generation is a job-specific problem. I do not think that a wide-spread encounter design change is an appropriate response to it. With or without boss displacement phases a paladin will suffer in this regard.

(5) The DPS is clearly at fault. A tank cannot prepare the end of the enmity combo for that transition and enmity is visible to all combatants even when the boss is displaced. This is not hidden information and the DPS could have waited accordingly. A dead DPS deals no damage.

(6) Ignoring what? I've been replying to all points you've presented more than once with the argument that future encounters are still only conjecture on our part and arguing change on content that does not yet exist is something I don't support. If Squeenix does end up making encounters that are tailored to fuck with healers, then sure, that's terrible. But there's no evidence supporting that future any more than there is evidence against it. Adding to that, current content does not have encounters that in my opinion warrant such a change.

(7) I'm afraid I don't understand what you're getting at with the examples given, or what you mean by "If 'management' and 'ability' was really a thing". The adds (assuming Goblin Snipers) are a piece of the encounter puzzle and easily defused threats. Bosses with pre-set enmity appear with no displacement phase in-between. They could just as well appear with zero enmity and quickly end up in the same place with a Provoke and enmity combo. Even if a healer or DPS generated initial snap enmity, a small change to Provoke timing would probably fix that problem.