Assume that you deploy a linked mailbox in a Microsoft Exchange Server 2013 forest, and the linked mailbox is associated with a disabled user account that's in a trusted account forest. When you run the Add-MailboxPermission cmdlet to grant permissions for the linked mailbox, the permissions are granted to the wrong account in the Exchange Server 2013 forest instead of the account in the account forest.
This issue occurs because of a known issue that occurs when Exchange Server 2013 takes user identities between the resource and the account forest.
To fix this issue, install Cumulative Update 10 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.