I’m encountering the same thing. also have oauth setup at auth.foobar.ai and the api is at foo.foobar.ai
Thanks for the tip on logging in via web, that works for me as well, but this isn’t a sustainable workaround for production users.
Note the login flow did work on mobile at some point, and was working until I cleared Safari cache.
chatGPT app version 1.2024.163 (9522135661)