On joining, the mission is coded to move a player into a new group using 'joinSilent' on the server.

The code looks like this:

["Joining player %1 to team %2", _player, _team] call BIS_fnc_logFormat;
[_player] joinSilent _team;

And the output in the log is this:

18:25:28 "mygamemode/BIS_fnc_log: [test_fnc_init_player] Joining player to team GREEN, with group B GREEN"
18:25:28 "mygamemode/BIS_fnc_log: [test_fnc_force_team_change] Joining player B Alpha 1-2:1 (AnonymisedName) REMOTE to team B GREEN"

However, on their client, they are in group 'GREEN', but on the server and every other client, they're in group 'B Alpha 1-2'

This is particularly unusual, because joinSilent was executed on the server, but they only move into the group successfully on their client.