Both the port and the ground, or in a differential port, the positive and the negative terminals, must be assigned to faces or edges of good conductors. These conductors may be 3D structures made of a conducting material, or they may be 2D structures with an impedance boundary.
When RF3p scans the patch to find the adjacent conductors, it ignores any 2D conductors that are interior to the patch. Thus, you can define a voltage between a trace and another conductor even if there are conductors between them, as long as the intervening conductors are 2D surfaces.
The patch itself does not represent a physical structure but rather just guidance for determining integration paths. Consequently, the material of the 2D patch is ignored by RF3p, and it therefore has no bearing on the results of the simulation.