I think this was on Sci-fi/ syfy channel. A teen visits a small town where a famous 50’s monster movie was filmed and it turns out that the alien monster is real and the movie is really a documentary. Somehow everyone but the original hero/star of the movie, now an old man, keeps developing amnesia and forgetting the monster’s periodic returns.
Monster! (1999)?
Found! Boy, even knowing the title and stars that’s a hard one to look up. Thanks!
Sounds like a Solve.