This is an old bug, reported on BI forum or mentioned as ancillary issue in other ticket.
If 2 players are joining (at start or JIP) on same vanilla weapon squad (group of 8), one player as leader, one player as marksman (not tested with others), the 6 AIs will now regroup on marksman at the leader global move order, then stay at marksman position whatever the leader's move order could be.
IMPORTANT: the move order must concern all units of the group. If individually ordered, there is no issue.
This problem seems to be ancillary because usually MP players barely play this configuration.... but I guess a far deeper issue due to the way subordinates obey orders. As other example, if your AI group (no second player here, so in SP as well) is spread on map and you order a move (as rally point or else), the AIs start to regroup themselves (on 2 or 4) no matter the distance separating them! So, if you want some efficiency, you need to order all AIs independently. IMHO, it's a waste, weird logic stuck somewhere in FSMs. (2nd ticket?)
Thanks for reading (and testing)