Hi @CAM
Thanks for the update!
Would a new test account with the same FirstName also be unsearchable?
If not and the issue is only with this user account, I would suggest creating a service request in Microsoft 365 admin center to involve support to look into this issue.
As to further troubleshoot this issue, we may need to capture the network packets of the search request to check if this is something wrong with the search process or with the user account.
Thanks for your understanding.
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