I’ve noticed this bug on Advanced with 5 people playing.
Those bosses that spawn after you deplete Rendain’s sheild but before the final fight. The bosses from previous campaign missions.
The point is, those bossed don’t trigger their phases until they are very low on health.
Example: Conservator starts summoning the shielding varelsi only at something like 20% health left.
This is obviously due to the transition between phases being tracked by a flat remaining hp instead of percentages (there is a lot more of hp on bosses on advanced).
I think this should be fixed.