OMG! He is just adorable!
And...he certainly doesn't look like a cat who has a problem.
Completely NOT what I was expecting to see.
What he is doing reminds me of a dog whose lip has got dry and 'stuck' to its' teeth, so it sort of grins that lip to move it and get the lip off the teeth. Oh, I'm not describing that well at all. It also kind of looks like an animal that is smelling a smell it doesn't like, BUT, your kitty isn't really making a true "Ugh!" kind of face. Another thing it looks like, is when I sometimes gently pet my cats' faces and when my hand 'drags' on their whiskers, I sometimes see them make that movement with their lip.
What he *really* reminds
me of, is my kitty Reilly.
Your kitty doesn't seem to even be aware of what it is doing, and Rei did a similar thing, too. Rei had a severe medical problem that got worse over time, but his back legs would sometimes quiver and shake like they had a charlie horse...but he *never* appeared to be in any pain...his legs would be stretched straight out with his toes spread and they'd just sort of quiver there. I could rub his legs and the muscles would relax...but what reminds me like your cat
...is Rei never seemed to be aware his legs were even *doing* something.
The thing Rei's legs did, didn't bother him at all. And it seems this 'grin' thing isn't bothering your cat at all, either...so I guess I wouldn't worry about it. Did he do it for the vet to see?
...this is a very odd thing!