Delve and Mercenaries create Invulnerable monsters

In Delve, I've been able to replicate this a few times.

If a rare monster focuses on your mercenary in the darkness, as in beyond the light of the cart while moving, the rare monster will become invulnerable to damage, as if they are still in the darkness.

This does not happen without a mercenary. If you leave the merc at home, go into the darkness with aggression from a rare monster, and then return to the cart path and light, the monster will be attackable.

If you do the same process or your mercenary lags behind into the darkness and a rare mob attacks then, then they will become invulnerable and unkillable.

I have tried returning to the darkness and using a torch to light the area, this has no effect.
I have tried using dynamite to do damage, this has no effect.

So far this only seems to affect the rare monsters that spawn while the cart is moving.