Me and my friend were fighting the skeleton boss and I had died but he revived me and as he did the boss teleported away and he didn’t appear again so we had to give up that run because we were stuck. 
#Boss bug?
twilit field
candid oasis
some people have reported this, we still need to investigate
hollow isle
Was looking if this happened to anybody else - this means we technically beat the game. Woohoo!
sharp flame
This also happened to me. It also teleported on the celling and we had to give up.
brittle bison
same just happened to me a my duo, he did his spike attack and teleported away and didn't come back, another issue we came across was those room where the gates close and enemies spawn, after beating the waves, my duo was still seeing the gates and collided with them, and was stuck in the room, but we Tp around to get around it.
celest oak
Same happened to me yesterday, got revived at same time we killed the beast, but we were just kinda stuck sitting around there