The iPhone “iPhone” cannot be synced. Received an unexpected response from the device.

Just got off the phone with an iOS tech and he was able to determine that there was a profile issue/mismatch on Apple's end that wasn't allowing iOS 14 public beta downgrade users to activate on iOS 13 clean install. I think they turned on a setting or allowed iOS 14 public beta profiles to activate now when downgrading to iOS 13. I am now able to pass the activation part of the setup without any problems.

I have the same issue after restoring to iOS 13 from the iOS 14 public beta on my iPhone 11 Pro. I tried a full erase of an older iPhone 4 and restore and it activates fine during the restore process. It's definitely not Apple's server that's not working/overloaded. Not sure what's going on with the iPhone 11 Pro.