If I have buccal fat reduction and some buccal fat is left behind, will it grow back if I gain weight (pregnancy weight etc)?

Buccalf fat pads are pockets of fat that can increase in size with weight gain.  If a patient that underwent partial buccal fat removal (so some buccal fat removal was left behind) gains weight, the results of the procedure will be masked by the weight gain. Therefore it is recommended to avoid gaining weight after buccal fat removal 

A buccal lipectomy removes a significant part of the main body of the fat pad but not 100% of it. But any growth of the small part that remains would be minimal and probably not cause any external visible change.