My contract had banked 2017 points into the 2018 use year. No problem. The seller had already banked the 2017 points ahead of time when I bought the contract.
Now supposedly if you're buying a contract with 2017 points, and those points were NOT banked before the closing of the banking windows, they will not allow you to bank them after the window has closed. That's my current understanding of the rules. BUT there have been multiple reports here of people getting an exception to that after closing a resale purchase.
If you buy direct, there is also an exception to the banking windows. I purchased 25 Feb UY points direct in November and was able to bank 2017 into 2018 with no issue.
Not sure if that helps or just makes it more confusing.