In certain cases , users will see this error in the request details page.
As per design of SCP 11.0, Support Rep of one Portal cannot be a Contact in another portal.
If a support rep in portal A , sends email to Portal B , (where he is associated or created) , then the ticket will be created under System Name.
We have plans to achieve this use case through feature ID - 2252 (in our product roadmap). Once it is released we will mention the same in our Product read me.