This can, and does happen. I remember one from a few years ago; the Miami Beach EDITION was slated to move from a Tier 3 to a Tier 4 with the annual category change. About six weeks after it moved to a Tier 4, it moved again to a Tier 5.
I don’t know any consolidated listing of these intra-year category changes. I think you have to just be the recipient of the unlucky surprise when it’s a hotel you want to book.
It’s pretty shady in my view. By Marriott going out of their way to publish annual category changes, it implies that’s the only time hotels change category. While the intra-year changes seem rare, they do happen and Marriott should be more forthcoming about it.