I don’t believe this is true, and I can’t find any confirmed examples of it happening.

There have been cases where users thought they lost a purchase, but it ended up being an issue where they changed storefronts to one where the purchase wasn’t available. The movie reappeared when they went back to the original storefront (and I believe even that behavior was a bug that’s been fixed).

I keep an eye on this because I’ve also purchased a lot of movies from iTunes.