No, this is not a bug with the last boss, even though that hasn't been fully fixed.
This bug happens in the large trash room after the second boss (trio). After engaging the first pull of trash, several portals will open. After the portals / mobs that spawn have been killed, a new wave of two Grievous Twilights and supporting adds should spawn, but sometimes they simply never spawn.
Without being able to kill the next wave of Twlights and adds, the door to proceed remains locked permanently, and nothing can be done to fix it except resetting the dungeon and starting over.
This is an ongoing bug that has been present for many months, if not years.
I can only speculate the cause - maybe the portals aren't killed fast enough and time out creating a glitch, maybe the adds are killed too quickly - your guess is as good as mine.
Please investigate and resolve this.
Edited by Lerozain on September 7, 2021 9:56PM