They legit outright nerfed it; it's even got Echo added to buff everyone.
This is a problem with tab-targeting large/tall bosses in general; if you cannot see their nameplate, it will sometimes not consider them 'seen' and will not target them.
Two alternative options:
- Bind 'Target Focus Target' to a hotkey, then focus-target the boss. As an added benefit this works to quickly re-target the thing you want to target in scenarios like, say, P3S where you have four Darkened Fire spawns that are invulnerable until you remove their buff. As a healer, if you need to switch targets to spot-heal someone and then try to tab back to the boss, half the time you will probably end up targeting a Darkened Fire instead.
- Bind something to Next/Previous in the aggro list. I have my mouse scroll wheel bound to this (and Control+Scroll Wheel for zoom in/out); I can thus just use the scroll wheel and zip my selection up and down the aggro list. And if there's only one enemy, as with the High Seraph, just scrolling the wheel at all will target them. For multiple enemies, with sufficient practice, you can get the muscle memory to know exactly how far to scroll to get X number of enemies lower/higher in the list. (As an added bonus, since the opposing team in Crystalline Conflict is treated as an aggro list and I have been doing this long enough -- e.g. two years -- to have that muscle memory, I can target an opponent almost instantly.)