My app has also been rejected for a similar reason:
The issue description states:
Issue Description
The app exhibited one or more bugs that would negatively impact App Store users.
Bug description: When attempting to Sign-in with Apple, your app loaded indefinitely.
Review device details:
- Device type: iPad Air (5th generation)
- OS version: iPadOS 18.1.1
I have tested the app using a freshly installed iPad simulator and found no issues with logging in. None of my testers have come across this problem so far. I am unsure how to proceed—it feels like something could be wrong with Apple's iPad testing devices or their network.
One thing that bothers me is that we do not control the Apple Sign-in process—it’s entirely managed by Apple’s UI and backend systems.