We are seeing this in a 3 player coop campaign where 1 player dropped earlier. The game was initially Franz, Louen, and Ku'gath. Franz had to drop a few turns in and then the Empire was wiped out. We continued with just Louen and Ku'gath.
We have been able to play plenty of battles just fine, but for some reason we are getting desync when Louen is fighting Grom. The desync happens around 10 minutes into the battle, just after the melee troops start fighting. This happened even if the other players are just spectating the battle.
I've attached the save file, however I needed to change the extension from .save_multiplayer to .save