When working with disconnected mailboxes I have found where the recently deleted/disabled mailbox doesn’t appear in the Admin Center.

From the exchange management shell you can run the following to find a mailbox, in this case I’m looking for a mailbox for Mark Taylor
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Get-MailboxDatabase | Get-MailboxStatistics | Where-Object {$_.DisplayName -like "*taylor*"} | fl displayname, itemcount, database, discon*, mailboxguid |
The information returned was the following:

As you can see the disconnect date and disconnect reason are blank.
I know the mailbox was on the database MDB01 so again from the shell you can run the following:
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Update-StoreMailboxState -Database MDB01 -Identity b31c95ca-5e40-4bdf-bf1b-9766521e0570 |
Now the results of the previous command show it was disabled and also the ECP shows the mailbox can be reconnected.


When the mailbox doesn’t have a disconnect date or reason it is still possible to reconnect it to the AD account from the exchange management shell by running:
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Connect-Mailbox -Identity b31c95ca-5e40-4bdf-bf1b-9766521e0570 -Database MDB01 -User mark.taylor |