Remove the full mailbox permission and add it back:
Remove-MailboxPermission -Identity <Mailbox > -User <Delegated User> -AccessRights FullAccess
then add back explicitly with automapping in Outlook:
Add-MailboxPermission -Identity <Mailbox> -User <Delegated User> -AccessRights FullAccess
It should show up after about 30 minutes in Outlook.