In your scenario of buying a September UY in July, does the buyer pay MF on the 2019 September points?
Since MFs are based on calendar year, Disney charges from date of ownership, so don’t think about points, but that,
So, if you buy in July, regardless of UY, you pay 6 months of the 2020 dues,
You will then be billed every January. When you buy, if you choose a UY that is still in its 2019 UY for points, you get those as The first set. If you choose a UY that is in its 2020 UY, then you get those as the first set.
The benefit of buying later in the UY it is like you are getting two years worth of point, but really nothing more than a strategic decision of when to buy.
Now, if you are buying a sold out resort, they may not sell you a resort at the end of the UY because they may not have those points to give. But, if you are buying RiV or AUL right now..current resorts in active sales...you won’t have a problem.
Once sales open back up, we will still be in 2019 Aug, Sept, Oct, and Dec UYs.