I implemented sign-in with apple on my ios application and it's been working perfectly on my end, whether it was run on a simulator or a real device or with the latest ios version (13.5). It also worked perfectly during internal beta-testing when I released it on TestFlight.
However, upon submission of my app to Apple, it was rejected for incompleteness by a quite contradictory error. When they attempted to sign in after choosing to hide or show email, an alert generated by the Sign-In with Apple system was presented stating that they were not connected to wifi, even though from their screenshot it would appear that they were.
I tried to reproduce the error in as many ways as possible on my end, using different combinations of wifi, VPN, and cellular. The ONLY way I was able to generate the "Could Not Connect" error was by actually not being connected to wifi or cellular.
If I had ever come across this error on my end, I would've dismissed as some connection confirmation error tried to sign in again, which I would assume to work the second time (if the error itself really was an error of some sort). However, this happened only on Apple's end, and when I asked the reviewer to attempt the sign-in one more time, making absolutely sure that they were properly connected to wifi, I did not receive a response.
My question is that is this an actual issue that occurs with Sign-In with Apple, and if so what would the solution be, or is this just a one-time glitch on Apple's end, which in that case I should just re-submit my app and hope that it doesn't occur again?
(I put this as a comment a few weeks ago by accident, setting it as the actual answer now)
Update, Problem solved! : A few days ago, I tried resubmitting the same app binary without actually changing the code of the build. The only thing I did change was the build number, (which I did by just by archiving and uploading the same xcode project as before but changing the build number). Surprisingly enough, it was approved! Nothing in my code had actually changed, so it just seems like the first time there was some glitch on the Reviewer's end, and the only way I could get the reviewer to try signing in again and avoiding this weird glitch was by resubmitting it as is.