@AselaDeCosta-5174
This issue my be caused if the RemoteRecipientType attribute is set incorrectly. If you just convert it back to shared mailboxes, the mailboxes may revert to regular mailboxes again the next time that directory synchronization runs.
We have to modify the RemoteRecipientType use the following command:
Set-ADUser -Identity ((Get-Recipient PrimarySmtpAddress).samaccountname) -Replace @{msExchRemoteRecipientType=100;msExchRecipientTypeDetails=34359738368}
You can check this article for more details: Shared mailboxes are unexpectedly converted to user mailboxes after directory synchronization runs in an Exchange hybrid deployment.
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