I have been seeing the same thing in my current game. I also have bombarded units with ships, planes, or artillery and weakened them significantly. Then when I move in to finish them off with my units, they are back to full strength and 100% morale. I've had some AI units get destroyed and then reappear at full strength in the same spot later than day. It's almost like the AI has started using gold or something. It's very frustrating to spend all day softening up targets with artillery, and then log on to find that all that work has been erased by the AI.
Edit: just happened again. Had a cruiser bombarding a tank in a coastal city all night and it was at 40% morale earlier this morning. Logged back in an hour later and it's at 100%. There is no IC in this city so I know it is not a new unit that just got built there.
