Cannot Delete Database due to unknown mailboxes

Hi team, 

I've run into a situation where I have moved all mailboxes (arb,archive etc.) but unable to delete the database. However, running the 'Get-MailboxDatabase <DB NAME> | Get-MailboxStatistics | Where {$_.DisconnectReason -ne $null}' shows me the below;

DisplayName                                                 MailboxGuid
-----------                                                 -----------
SystemMailbox{a0d85ace-9060-45a9-9f94-fe7e9d5020cd}         0bb17fc5-d6de-4e59-a9a5-3c43368f2f7b
HealthMailbox-<server-name-db-name>                    fa981ae2-c98b-4372-82c2-2cb7684e7c73

..... there are 14 more like these.......

Something to note is that the 'DisconnectReason' is blank/empty.

DisplayName      : In-Place Archive - HealthMailbox-<servername>
DisconnectReason :
DisconnectDate   :
MailboxGuid      : 69829e12-f7e9-437c-9a7c-82bb1cs8aa39
Database         : MDB01

Running the below command also does nothing to the above

Update-StoreMailboxState -Database MDB01 -Identity 69829e12-f7e9-437c-9a7c-82bb1cs8aa39

How can I get these removed? What are the impacts in the deletion method?

*System mailboxes(Arbitration) are already in a different database.

Thank you.

Jude