Only the first player who enters an APC/Tank can pop the smoke defences. For exemple if a tank has 2 players inside, one is driver and one is set to command/gun in multiplayer. The commander/gunner has to jump in first as commander in order to be able to use smokes. If later when tank coming to a stop, the driver decides to go as commander while the other player is in gunner spot, the smoke ability will disapear for the commander, making it impossible to smoke (watch top right corner and you will see smoke ability disapearing).
Only step to zero this bug is all players have to leave the tank and let the commander jump in first. This zero mechanism must be done every time a new player switches to commander.