It's been quirky that way with the dates since it started I think. I've modified a couple of reservations in the past week and it is quite disconcerting to say the least! I first noticed back when doing some modifying for our past April trip when I would change the search in modifying and the points were suddenly coming out higher. It's not real good for it to be inconsistent like that.
The messages - it's also something that's been going on since one of their big updates last year although back then if you had something that didn't work (like searching just a bit before the booking window opened or if you did select something that you weren't eligible to book) then the message was that the room you were looking for was no longer available. Now it's the Riviera message. It's like they can only have one message at a time.