I have had the same experience. I think the problem is specific to iOS 14.
This is as far as I went testing Apple Sign In using the 'Juice App'.
Running on simulator iPhone 8 Plus iOS 14. FAILED
Running on simulator iPhone 8 Plus iOS 13.5. SUCCESS
Running on device iPhone 8 Plus iOS 14. FAILED
Running on device iPhone 8 Plus iOS 13.7. SUCCESS
Note that there have been absolutely no changes in the source or project configuration.
All failing on the same step. i.e when device is authenticating my credential. The spinner spinning indefinitely.
I only have two iOS devices. I hope someone else can expand on this result further for the benefit of everyone affected
bluearth
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