Breach cannot be completed due to mandatory rare mob stuck in terrain

I encountered a bug where I was unable to complete the Breach event.
That mandatory rare mob is stuck in the center tile and cannot be defeated and consequently I'm unable to close this Breach.

The mob is not only stuck but also untargetable, I couldn't apply Assassin's Mark to it, suggesting this is a genuine bug, not an issue with my Totem's targeting priority rule.

I had the same thing happen today. On the same map, in fact. Wasn't able to complete the breach because a single mob was entirely invulnerable