Citadel boss phases bugging

Here's what happens:
-Fight starts
-Damage the boss until they should phase
-Boss becomes invulnerable and use 1-2 more abilities back to back before phasing
-It seems like these extra abilities do more damage than usual, I can tank many citadel boss hits but these are consistently 1-shotting me

Note: this happens when I deal a LOT of damage very early in the fight and the boss would phase quickly