Fixing Co-op Desync & or how to update mission to latest version:
To fix this issue go to your mowas2 dir folder...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods

Find the folder named "the war machine v1.2". And Delete it.

Then, come back to "the war machine v1.2" workshop page and "Unsubscribe" and then "Subscribe" again.

Doing this forces an update of the downloaded workshop content tucked away in...
C:\Program Files (x86)\Steam\steamapps\workshop\content

When you open the game again (if you had "the war machine v1.2" mod activaterd), you will get a message effectively saying it cannot be loaded. Simply go to options mods and reactivate the mod which will copy the newly downloaded content into...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods

All parties playing co-op should follow these steps to ensure everyone has the same version.