At least, I hope it is only a visual bug.
When an AI unit is engaged in melee, it visibly gains morale when it inflicts damage on my units (this is normal). However, I have been noticing on a reasonably regular basis that the AI unit health bar looks like it increases at the same time (not normal!).
This seems to happen most regularly with dwarf HI.
As it is an instantaneous thing it is hard to screen capture, but I'll see what I can do.