I noticed reports about issues with the game master module. Today i experienced the problem myself and did some tests to investigate the problem and create the best workaround.
The problem occurs when game master modules are placed in the editor with a valid owner property. If one of the units is occupied by a player who joins while the mission is already running the assignment of the curator module does not work correctly.
In my tests I always used C_Soldier_VR_F (VR Entity) as units but according to other reports the issues seems independent of the unit type used.
I also used multiple game master modules so i can not confirm if the issue occurs on a mission with a single game master module.
The issue seems to be in the game at least since 12/2016 but i could not find any bug reports on the feedback tracker related to zeus and JIP using the search.
If you need more information let me know.