You don’t specify how long ago you had your facelift. Nerve injuries are very common with facelift surgery, and luckily the vast majority of them heal on their own in 6 or at times 12 weeks. Your asymmetry is most likely due to swelling, bruising or compression of one of the buccal branches of the facial nerve that innervates that lip elevator muscle. And yes, I have seen this type of injury and far worse. Time ought to improve this. In the meantime, I would exercise your lip to help wake up the muscle, or you could try a tiny, and I mean tiny, amount of Botox to the opposite side just for balance while you wait for the muscle and or nerve to heal.