If it’s been over tightened or used with a post too small then the opening of the seat tube will indeed be too small for a 27.2 but that’s not to say that further down it isn’t.
Check the split in the tube at the back and make sure both sides of it are straight.
what I can say is when the bolt is tightened the metal of the lug is very close together. not sure how normal that is though. 27.0 does seem a very snug fit though.
pretty sure. 27.2 wouldn't go near it. will double check and give it all a good clean up, but assuming that isn't the issue?
it will be slipping about a cm or two over the course of 3/4 short commutes (3 miles or so each way)