When migrating a shared mailbox, the option 'Migrate Mailbox Delegation - Enable Automapping for shared mailboxes' is used during the migration task.
The source shared mailbox has both direct user and security group permissions. After migration the shared mailbox is added to the mailbox of the users who are directly assigned permissions. The users assigned permissions through security groups do not have the shared mailbox assigned to the account.
This is the expected behaviour. Automapping is set on the user's mailbox, not the shared mailbox. This means if you try to use a security group to manage who has access to the shared mailbox, automapping won't work.
The users in the security group will need to have the shared mailbox manually added to their account. Or grant explicit permissions on the source shared mailbox.
