It's quite a subjective word ("fake") that means something different to a lot of people, but...
For what it's worth, I showed some snapshots of Fi snarling smiles to a friend of mine (who is not into visual reading) and he said it looked like they were giving off fake smiles. I think what he might have been identifying is a type of counter-tension.
In Gamma types, the clash between Fi's upward tension (going up toward the eyes) and Se's reluctance to yield to that upward tension (due to their sharpness), makes it look like "the smile is not going into the eyes: therefore it's fake" if that's a conventional wisdom they follow.
I could see how it could be taken as a "forced" smile... But it's not usually the case. I find Fi types smile authentically more often than your average Fe users.