units in mission editor, not appearing in game

[super-truite]

I am making a dogfight mission with some AI.
Some of the AI units I place on the editor are not present in game when I launch my server with the mission.
Also sometimes, the mission briefing does not show up in game. I saved several time the mission and did a cleanup of the server mission files to be sure 
I am not loading a corrupted or old mission. Also, the units are not disabled and are not spawned. 
The integrity check is not finding anything.
This must be a silly mistake on my part, but I am running out of ideas.

I will do a thorough diagnostic of the mission by removing stuff and trying to find a minimal failing example, but in the mean time I thought it would 
be wise to ask here as this might be some common mistake.

Mission files attached. For instance, the tanks in the "Landing area" group are not showing up in game.  One artillery group is not showing up as well.

mission.zip

[super-truite]

OK I understand why: all those units are on the ice (lakes or Volga). It seems they go under water ...
Unfortunately I have to spawn some units there. Maybe I will try to make them move on ice with waypoints to avoid the bug.

[JimTM]

Also, somehow the object id and entity id associated with (at least one) of the tanks went missing from the .mission file for some reason. When I saved the mission, the correct object id and entity id showed up. I did not test the mission, so your river scenario may also be a problem. The object id and entity id are explained under "ID" on pg. 219 of the manual. 

[super-truite]

thx, I will check that, but I did some tests and tanks are indeed disappearing if spawned on ice and they cannot follow waypoints all the time when being asked to go on ice.
I wanted to make Ju-52s land near or inside the city, but they can't take off after landing, so I was forced to put my landing zone on ice and now, the  tanks that are acting up ?.
I guess I will have to change the way the mission works !

Edited May 1, 2020 by super-truite