In my experience, the time for ROTR doesn’t change after you buy it. The time you are able to buy may change from what you select, but if you elect to purchase that is the time you get.
For example, on the Tip Board the time shown will presumably be the earliest return time available at that time. If you click on that time, you will be offered the chance to choose a return time. By then the earliest available return time might be later than what you clicked on. Then when you select a time to purchase, and proceed from there, the time offered might be later than what you chose. But, if you then click to purchase the time offered, and proceed to payment, you should get that time.
The important thing is to always check what time is being offered and not to assume that you will get what you first clicked on.
If you pay and the time you get isn’t something you can use, I would go to one of the Guest Experience Team umbrellas and see if they can help you get a different time or five you a refund.