Simulacrum wave won't finish because Kosis is out of the arena

As title says.

I was in the last rounds of Simulacrum T3 in Clearfell Lumbermill (zone lvl 81). Kosis' icon was where the red circle is in the picture then disappeared.

Things that happened before the bug and might have caused this :

I died twice very fast.
Both bosses lost sight of me after second death.
I killed the other boss very quickly while being out of sight of Kosis.

Follow-up



I left the simulacrum and reentered. Kosis is still alive but now the description for the next wave is there. I can't start the final wave though. RIP