I've noticed that as well. It's annoying when you want to cast an arcane tower on the same spot, but it won't let you because the square must be empty. Even if the enemy is killed and there's no ghost remaining, the square is still non-empty for purposes of casting arcane tower. Picky tower builders don't want to work on bloody fields???
Also, I've noticed AI champions that have regeneration can't be killed with melee weapons. After the won battle report screen, there they are, still kicking, albeit with only a single hit point. At one point, I was on a road and managed 4-5 attacks on the same regening enemy. At each death, they'd be one square further away. Since it was a road, I had sufficent movement points left to attack mulitple times. I didn't mind, they were an easy experience point generator!
I suspect there's an evaluation order difference between melee and magic attacks relative to regeneration, as I couldn't kill the regening AI champion with melee attacks, even if the attack would've left him with -10 hp, they'd still come back for more. But a flame dart that took him to -5 was sufficeint to kill them permanently.