The boss disappeared

I was playing through the excavation area on my 4th act***. I died to a boss, and then the boss died. He died after I died, and when I tried again, he just flew through the textures in an unknown direction. I tried respawning at a checkpoint and restarting the area, but nothing helped—the boss simply wasn't there.

It looks like the boss appears with a script after death and just flies away somewhere