Vaal Crossbow guy bugs out

Playing Spark Stormweaver. For some reason, this mob (vaal guy with a ball for legs, and shoots 2 crossbows fast) always bugs out when it becomes a rare monster to kill. On death, the monster becomes untargetable but still alive, following me for the rest of the map pelting me with arrows and I can't finish the map. Cannot replicate this bug on other characters, for some reason only happens with this mob and with Spark.

My other friend who also plays Spark has had this happen to him 3 times. For me, twice.