Unprocessible entity error when trying to add user to team

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When attempting to add certain users to teams, there are a few users that have consistently been failing with status code 422, with the given error type of invalid and message The request could not be processed. The message doesn't line up with the one stated in the API docs for updating team membership, which the Troubleshooting guide indicates I should refer to. I've checked the users that aren't syncing properly, and they are all active users and not organizations, which is the only stated reason the 422 should be occurring.

Has anyone else seen this kind of error when managing teams through the GitHub API?