Happens in Mortal Empires, with any quest-related teleport battle being started. The multiplayer would load into the start of the battle but after loading the battlefield, both the players would disconnect with the message that there was a desync in the game. We've tried verifying game cache multiple times, sending host save files to the other player and restarting the campaign multiple times, but even rushing the quest battle would provide the same results of desyncing.
I recall us being able to try the quest battles on the Vortex campaign map, but as soon as we made the switch to Mortal Empires, neither of us has been able to try any quest related battles without the desync problem occurring.
We're playing on the latest build, use no mods and whatnot.