Happened with me against arkhan earlier, no idea what causes it. Might also just be a vanilla bug.

Its vanilla bug, one unit model stuck somewhere on the map.

I've noticed it happens mainly with Undeads, who tend to disassemple instead of retreat. Each time I had this bug it might be caused by my cavalry charges. I am pretty sure when I was charging the arriving reinforcements with my cavalry having about 120 charge stat - some of enemy infantry just flew away from the map.

Im playing as a Shadow Legion and every fight with Vampire counts are like that, im not throwing units behind the map. After winning by time limit, im losing a fight and get robbed by enemy without any reward, but enemy get almost all units "rise by the dead" 5/6 of whole army. It's realy anoing to wait until time goes one, and lose items all the time.