It’s happened three times in the past week in Dread Cellar to my group as well - ruined a couple of otherwise no-death-yet runs (tho we did get the tri eventually).
The door after the first boltwyrms is impossible to open for some reason.
I’ve never seen it happen before the last week or two. I wonder if I just didn’t run it enough (although my sticker book was full months ago) or if something has changed with the game/performance where triggers in dungeons are not going off properly.