Sometimes, the spirits of Exorcism get stuck. They just spin around some point and can no longer attack. The only way to fix them is to move far away from them so that they respawn.

No specific repro for this, you just cast Exorcism and it just happens randomly to some spirits.

I'm assuming this happens when their randomly chosen point to move to is unreachable for them due to how they can only turn in big arcs.