If anything will make me rage-quit it is bosses becoming invulnerable

It is such bullshit that they can keep fighting after becoming invulnerable. Jamanra hit the HP wall to transition, but instead decided he would keep fighting for another 10 seconds (unexpectedly) instead of transitioning.

Random bugs like this is why I made suggestions around failed citadels regenerating over time or even having multiple attempts.

Sadly my bro all the bosses who have "stages" do this in Poe1 and 2 - unsure why I guess they can't code well enough to tell it that when it hits the HP mark to move stages to stop moving. Can't count the deaths I've had over the years in 1 and 2 because I forgot this happened, hah.

Its really funny that the mediocre bosses in this game are the "least bad" aspect and people just kind of run with this premise that the game has good bosses.

Like the act 1 boss was mildly interesting so people clung to this COPE that "well well uhh... at least the bosses are epic"

Nothing about this game succeeds past mediocrity.

Similar problem with the elite enemies while running maps. Stun, it, knock it to the ground and while immobilised it still keeps pumping out mechanics. Whats the point of playing a stun build if the stun doesn't interrupt the mechanics?

100% agree. This is a problem as old as the original game, and in fact I would argue that uncontrollable boss invulnerability is a fundamental flaw in any game design.

My most pronounced memory of PoE1 was the time I managed to get a mirror on lock for a sanctum completion. Unfortuantely, I pushed Lycia to 0% health instantly in the starting room (she is supposed to transition immediately at 50%), and instead of transitioning as soon as she was done her current animation, she decided to start a beidat's gaze super smash attack from scratch after already hitting 0%. It was in a spot where it was literally impossible to survive, since she was up against a wall and the only place to get behind line of sight was within the blood (which i did not have the resolve to sit in through an entire beidat's gaze channel). On top of everything, it crit (since I knew the character can survive the hit once from full health with his potions running, all things that were true), and it one shot me.

It would have been my first mirror ever, and removed by a bug as old as the first boss to have invulnerable phases in open beta (Dominus if I recall correctly).

TL:DR, if a boss is invulnerable, it should immediately drop everything including actions in progress and get on with doing the thing that it is invulnerable for; but otherwise, bosses should NEVER go invulnerable in a situation where the player has no control over breaking the invulnerability (Tukohama is an acceptable invulnerable boss phase design, Jamanra is not).