Bots do that sometimes. When I leave my dedi LAN server I use occasionally the bots reset, change to the default blockhead colour despite appearance events, and ignore anything you tell them to do.
The host rejoining reverts the change, but you have to respawn all the bots.
Alternatively, you could try recreating the bot spawns once the host has left, since the new blocks won't be the same as the previous ones and that might make them behave properly.
Beyond that, I have no idea. It happens. :(
