I am having this issue as well with some of the animated thumbnails for a handful of my games. Notably, the cover image for a game I released a few months ago animates correctly (https://turboshook.itch.io/tomb-of-the-underking), but every other animated cover image is broken.
I hope we get some clarity around whether this is a technical issue or if there are new size restrictions for gifs (or something along those lines).