Simulachrum wave not finishing

Doing a Simulachrum, one wave did not finish. Killed all monsters and bosses, but from the looks it felt like some monsters were somehow frozen at spawn-point or sth. (not targetable, but visible on screen in some sense) and thus could not be killed. (Thats just my guess)

Simulachrum before went fine, so occurence seems to be random.
Left the Simulachrum after about 30 sec, came back in, then wave seems to be reset but i cannot click wave starter.

Bug-Reference ID 224.630.759

Edit:
Found a mob hiding somewhere, finished.
Sorry for bothering :)