Mismatch between exchange recipients list and mailboxes set up in 365 Admin?

Did you check under Resources in the Exchange Admin Center? Not all mailbox types are shown under the Mailboxes page, and there can be some scenarios in which assigning a license to a user does not result in provisioning a mailbox. But if you are seeing mailbox usage data in the M365 Admin center, likely there is a mailbox indeed, which we can confirm via PowerShell. Connect to ExO PowerShell first, then run the following cmdlet:

Get-Recipient email address removed for privacy reasons | select DisplayName,PrimarySmtpAddress,RecipientTypeDetails

This should find any matching objects in Exchange Online, and also give you the type of mailbox/recipient. If you are not getting any results, try running the cmdlet again, this time providing the UserPrincipalName as input. If not getting any matches again, we might have to check your permissions.