We had the same experience as OP about a few weeks ago when we bought Riviera direct. In our case, we were open to August, September, and October. We communicated what we were told by the wonderful folks here about extensions of banking deadlines for Aug/Sept, and were told that was not possible and October was the only possible option to get 2020 points and bank them at the moment because we were still not past the May 31 banking window for that UY. For us, it was fine because even though there are occasions we book in summer, we much prefer to go at a time with cooler temps (fall/early in the year), so we didn't mind too much.
To understand the policy, I needed only to see the inventory when we tried to book our Welcome Home trip! Availability right now is super scarce, even with the "welcome home" advantage (and even that is not much of an advantage--the wait for a studio was so long, we ended up just booking a short 3-day trip ourselves in a 1br preferred view and sucking up the point cost on the only dates in 2021 we could find for our resort).
I get it seems unfair because it was a perk that used to be available to direct buyers, but this actually makes total sense to me and I think is the right thing for
DVC to do for its existing owners. It would far be more unfair if people who couldn't travel last year or even this year (e.g. international guests subject to travel bans) have their 2020 points expire because they can't book anything. Especially since the existing owners, unlike us new buyers, actually paid dues on those 2020 points.
I think this is why they are not doing any advance sales for the new Grand Floridian this year. Waiting for early 2022 so they don't add even more "retroactive" points into a year already overstuffed with expiring points.