Probably unrelated: First lab12 run (yesterday) with Gurgare I mixed up curse and recall shortkey in Boss Room. So I left after 5 seconds myself (accidental recall).

Had to do lab12 again. About 20 min ago (around 14.30 CET), I was in Boss Room again. My GC started to hit him, I lost focus on a curse and then the boss vanished.
I was left alone in the room. No time out possible, I was there for about 5 seconds.

Just finished next run, boss was there and died as he should
So no reoccuring issue for me.