I am experiencing an error where AI team being led by the player will not get into the vehicle, when player is a commander or passenger (through Ride in back option).
The infantry that is not in the vehicle will start walking to the positions from where they go straight into the vehicle, but instead of mounting they will just wait there. Player has to leave the vehicle to let them go inside.
It doesn't matter how big the squad is or if the vehicle is only with the player or with him and part of the team, it seems that whether the AI squad is "attached" from the mission start or added during the scenario does not matter either. Terrain also has no impact on that. Switching seats has no effect.
However the AI will mount the vehicle if player is not a leader, no matter if he's outside or already in the BTR.