Pipes / ducts / etc export using system material not actual material

If I click on a pipe and check its instance properties, I see a "Material" property with something like "PVC" or "Copper" set as a material. These are legit materials and found in the Materials browser / manager.

However, let's say these pipes are part of a system called "SEWER". If "SEWER" as a Pipe System has a material set (i.e. not "By Category"). Then, that material will be used, not the actual material of the pipes / ducts / etc.

How do I get the pipe to export using its actual material, not the system material?

This in practice has multiple problems, firstly a lot of MEP users model using the system material as something like "SEWER" which means pipes end up having materials named after systems, like "SEWER", "HOT WATER", etc instead of "PVC" or "Copper". Also, some systems may have pipes with different materials in them (like portions being PVC, other pipes being HDPE).

No source file, test, or entry point is named. Reproduce the export with a pipe system material and different per-pipe materials, then trace the IFC material assignment; done when exported pipes use their actual materials, including systems containing mixed materials.

Written by the indexing model from the issue text.