I hate to speculate, but this was working with iOS 14, just before WWDC2020. Then they announced a security vulnerability that was discovered with sign-in with Apple and the keychain, and immediately following that announcement, the simulator Sign-In with Apple would just hang with an infinite spinner. Others have reported that the TFA is the culprit, so perhaps there is something to that and the backdoor that they discovered. Let's hope they figure out a way to patch it up. Edit: Please upvote this...

Same problem here. I've read that a ticket has been filed, and the workaround is to set a state variable. This seems wonky and potentially working around some reasoning that apple has for calling it more than once, which they don't indicate why.