I had this happen yesterday on both the web site and the app. I had to call in to get them to apply the PP. It just showed the upgrade screen with zero options shown (miles, paid, or PP).

The website seemed to have serious issues, and getting my wife's award ticket changed took an hour-long call to reservations, where the agent had to go to her help desk 3 times to get it to actually reissue; at that point, I couldn't even sign in to the website or the app (they would ask for password or in the case of the app, biometrics, would take them, then just go back to asking me to log in again).