I have seen something similar in the past too, where creatures (mole cricket, other ants, and some others) fail to pick a new target when their initial target is either too far away or out of reach. (So I think it is a creature issue?)
I am guessing this is something related to setting a new attack target when one has already been selected. (Because creatures never fail to select their initial target when entering combat, so I think it is unlikely for something to go wrong there?)