Port face selector source object is not unique.
A port face selector is an internal intermediary object between a structural slab and one of its bounding structural members.
Currently, the source object of a port face selector is limited to a structural member.
A relationship between a port face selector and its source object is instantiated more than one time, which can be the result of an automation problem or a copy and paste problem.
The slab limited by this member will not correctly evaluate boundary offsets.
What You Should Do
Because this object is internal, there is nothing that you can do directly.