I have actually seen something similar happening to my imp once.

I was fighting [Underdark spoilers]

the 2 Minotaurs. Then the Bullet burrowed into the fight, while I killed the minotaur next to my party.

That completely bugged out my imp, who tried getting into combat but failed. In my case only redoing the fight fixed it. I thought this bug was fixed already, but apparently not.