I experienced the same issue with XCode 13 GM and finally found the cause: during the process of uploading the archive, you are presented with a dialog that includes a "Manage Version and Build Number" checkbox. By default, it is ticked (i.e. checked). Untick this box to prevent the build number from being auto-managed by Apple. I'm not sure whether this is a new field or the default setting changed but XCode 13 GM is my first experience with this issue.
Now the question is how to get newly uploaded builds to be recognized by TestFlight, as my manually-managed build number is less than the last auto-managed number. This seems to prevent beta users from being notified of these newer builds even after I expired the earlier builds.
NealK
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