Hello! The following seems to be a reoccurring issue in multiplayer games.
Bug: Undead army ignores living army. The combat preparation timer (and the combat itself) doesn't start. The zombies and the living coexist in the very same spot for a while. After that the undead army marches away like nothing happened.
Possible cause: Unknown. (I have a suspicion that the Wolf Chariot's preemptive strike is the culprit, but that's just a guess.)
Temporary solution: Deploying a fresh unit at the same spot where the two army "chill" together. That will trigger the 1 hour timer.
Worse scenario (didn't occur to me personally): The two armies pass through each other. Obviously, in that case the former method won't work.