When you assemble previously disassembled vehicle that is not local to you, the vehicle passed into WeaponAssembled event handler doesn't have a group or crew in it yet, and its somewhat problematic to capture it by checking the vehicle on next frames as it depends on network latency of vehicle owner. The units are actually there in such case, just outside of the vehicle, and to get them you need to do a hack of iterating through all entities of UAV crew class and select ones that have assignedVehicle to EH-provided vehicle.

I suggest to extend WeaponAssembled event handler and provide created UAV AI units in it. Since the game is hardcoded to only create UAV driver and gunner, just have them one after another in the event handler or two nulls if no UAV units were created.

this addEventHandler ["WeaponAssembled", {
	params ["_unit", "_staticWeapon", "_uavDriver", "_uavGunner"];
}];

I can make a repro for this situation but I don't think its really needed, if both driver and gunner were provided by event handler, it would solve it.