It has now happened to me several times-- mostly in NYC, but also recently at the Hyatt Regency Churchill -- that I check my reservation in the App right before the cancellation window ends and find that the room description has changed to an upgraded room. In these cases, it has been a modest upgrade -- like from a street view to a garden view room.

However, when I check-in, I find the "upgrade" is magically gone and I'm back in the base rate room. This has happened enough times that I question whether it is a coincidence. To me it's a serious thing, because In some cases these phantom upgrades dissuade me from re-booking elsewhere.