I remember this being an issue before but it should have been resolved. Could you please enable Debug Logging, analyze the item, wait for it to finish, then download your PMS logs and share them with me, please?
Yep, here they are.
So in this case, the movie already had the thumbnails (checked before), I analyzed it, and it started generating them again. Once done, if I clicked Analyse again, it generated again…
And I added a new movie and the thumbnails were not generated.