Bonvoy has changed its rules and it works the same way Hilton does -- If you need to cancel one night of a multi-night reservation, you must re-book in its entirety and you risk not having the nights you want available. Marriott Bonvoy has been doing this for at least a year, so I am at a loss when you say Marriott never does this -- they do, and have been for quite some time now.

The only chain that I am certain that permits you to do this now is Hyatt, but with Hyatt you never benefit from a reduction in points on multi-night stays, thus unless there is a minimum of nights needed to book a room, one should always book one night stays.

I wish I could say that I empathize with you but the rules have been clear in both frequency programs that you have cited, that you run the risk of having certain nights evaporate on you at a certain rate if you wish to modify a multi-night reservation. The mystery to me is why some seem to think that the hotel chains are in the wrong, when they clearly are in the right with respect to guidance concerning their respective rules governing their programs.