Too much spellspeed screwing up AOE-rotation.

[ShinryuReishiki]

But the problem does not stop there, and this might be a bug. The first manatick normally gives me 1192 mana, but when the fire 3 cast finishes right before the 2nd manatick, giving me just the mana from the 1st tick, I do <not> end up with 1192 mana, but with just 1000. Yes, exactly one thousand mana.

If the MP tick happens late in a cast (~ last 0.5s) which would remove Umbral Ice, you end up with an MP tick as if you had neither Umbral Ice nor Astral Fire up. This is done in order to prevent us from using Transpose > Flare > Flare.

After you Transpose, you get 1192 MP from an MP tick in Umbral Ice 1, then you spend 266 casting Fire 3, putting you at 926. You receive the second MP tick late during the Fire 3 cast, so it only gives you 74 MP, putting you at 1000 MP.

I'm running 593 Spell Speed on my dungeon set, and this is what works for me personally. After I Transpose, I don't mash Fire 3 right away. As soon as I see the MP tick (I just watch for a color change in the MP bar), then I hit Fire 3. Just the delay from human reaction time (0.2~0.25s) is enough to ensure a second full MP tick, which leaves you with enough MP to Fire 2 twice and Flare. Since the rotation is dependent upon Transpose's cooldown, waiting that fraction of a second doesn't have any impact upon your DPS.

This isn't a problem with spell speed, but rather the timing you get with the server MP regen ticks.

It's both. People that run lower Spell Speed values typically never encounter this.