I had a hunch on a repro for a stuck in combat bug and this thread prompted me to test it. It turns out that you can fairly reliably repro if you damage a mob, leash it, and then kill it. This seems to fit with the OP’s warg killing scenario because for these mobs it’s often necessary to leash and reset to take out the adds.
I believe, in the past, I may have also reprod by polymorphing a mob, killing another, leashing to reset combat, and then killing the mob that had been polymorphed.