It seems there are occasions when towers just miss hitting anything at all, even ballistas. Is this supposed to be another freaking 'random' feature or is it a bug?

Also, there's one enemy from early game that frequently gets ignored no matter what priorities you set, until he's the only enemy in range. He's not very strong overall, but in the early rounds of a new campaign he sometimes slips through because he needs multiple hits and the ballistas out front have used their shots on the weakest targets even when set to prioritize most health. Does this guy have special targeting rules- an undocumented stealth feature, perhaps? Or is it just a bug?