I'm having a very similar problem. My credentials work in a bunch of other devices, just this iphone Xs Max that is cousing problems. When I try to do the in app purchase, it asks for my password twice, says I'm all set and they I get stuck on a loading screen after that. If I close and open again the app, I see that the purchase didn't go throug. I tried deleting and reinstalling it from testflight, many different credentials on the system and sandbox... nothing works.