Convert on Death map mechanic failed to convert one mob

Fungal Hollow map, Flame Hellions set to convert on death. A single Flame Hellion (normal/white type) failed to convert properly. Continued to chase me and attack me (and damage me) in the area. Other converted Flame Hellions ignored it as if it did not exist.
I was able to run far enough away to get out of leash range, but returning to the area made it chase me again. It also attacked my ballista totems.

Portalling out and running a Syndicate Lab in the map did not affect it in any way.