Converting Source Mailbox to Linked Mailbox necessary?


we are currently working on a forest to forest migration process, with sidhistory in place, so migrated user still can access resources in the source forest.

The mailboxes will also reside still in the source forest.

For the migrated user the access to mailbox is working partially. it can use OWA with no issues, but in outlook some features are not available like, Out-Of-Office

 Settings, Serach and MailTips.

If we convert the Mailbox to Linked Mailbox, everything is working fine as expected.

Is it essential that mailbox should be converted, although SIDHistory is active?