The bug is at the execution phase of the last boss when 15% health left.
The boss explodes a few times and all adds (Mage), mines and blackhole AoE should despawn.
The bug comes at this phase:
Adds do NOT despawn
Blackhole stays sometimes (no new ones spawn, but old ones do NOT despawn)
This can be very annoying, because as far as I remember it does despawn.
Greets,
V