Mini Bosses Disappear After I Respawn

Twice now (once in the 4 Assassins side quest and once in the Symbiotic side quest) I had nearly defeated the mini bosses before being killed. I respond and followed the diamond to where the mini bosses supposedly were but nothing was there. Is there any way to fix this glitch?

It’s possible they’d died from DoT while you were respawning. Did you see the echo lying where the boss had been? Otherwise, all you can do is restart your game.