While it may or may not be directly related, I had something similar happen to me with a cash booking. I was changing flights on on a BS fare. There was no difference in price and plenty of availability on the new flight. When I would get all the way to confirm the change, I was receiving an error that the price was no longer available. I tried several times (both via the app and the website) over several hours and kept receiving the same error. The price never displayed a difference on the screen.
i called WN CS and the agent didn't see an issue. He was able to book the change for me quickly on the phone.
i wonder if there isn't a glitch causing some fares to fluctuate by a couple of pennies . You'd never see it on a cash booking as they display the value to the nearest dollar, but the issues you describe with booking on points may explain what is going on.