The following phenomenon is occurring on iOS15 beta 4. (I didn't check in other iOS 15 beta versions.)
Immediately after completing the NFC scan, when the App ran nfcReaderSession?.begin(), tagReaderSession(:didInvalidateWithError:) was called and the error code was 200. Immediately after that, tagReaderSessionDidBecomeActive(:) was called and the scan sheet was displayed.
The [Cancel] button in the scan sheet is not operated.
If you execute the method after a while after the NFC scan is completed, the above phenomenon will not occur.