I'm getting this issue too.
I found this thread which could be related: https://developer.apple.com/forums/thread/702988
The following two lines on one of the comments (https://developer.apple.com/forums/thread/702988?answerId=711560022#711560022) might be relevant here:
This error occurred because a different Apple ID accepted the tester invitation for ***@.com, and a tester can be linked to only one Apple ID.
Note: Testers in the Deleted state aren't fully removed from TestFlight until their last installed build expires.
JamesAsquith
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