Basicaly,Im doing nothink-if you didnt use action of spellcaster or ranged unit,it cast spell or shoot to random unit after you end your turn. If mumies do it,they convert. They dont have such spell. So its probably not intended.
As for Magic Missle it is normal i think to be able to attack corpses or buildings. but if it can attack TC or neutral immunitive units (stones/statues/fires) than it is a bug.