Shared mailboxes are unexpectedly converted to user mailboxes after directory synchronization runs in an Exchange hybrid deployment

There is random issue that can affect Microsoft 365 shared mailboxes if you have a synced user account that has been convert within the 365 portal to a shared mail box.

Where it will dismount the shared mailbox and convert the user back in to a user within 365.  the fix is to change the following AD attributes with AD.

msExchRecipientTypeDetails 34359738368
msExchRemoteRecipientType 100

Let the AD sync to 365 and then give the user an Exchange 365 license and this should re-join the mailbox.  Once that's done convert back in to a shared and it should no longer randomly disappear.

Microsoft KB2710029