How to use physical material masks (or otherwise make composite materials behave)

I am working on a system for unique bullet impacts depending on the material hit. I have been successfully utilizing physical materials so far, but many complex assets only use one material slot for everything, which means I can only assign one physical material, even if the object is made of both wood and metal, for example.

I tried physical material masks, which were implemented for exactly this use-case, but for some odd reason they are limited to master materials. Any material instance seems to ignore the mask and there is no option for changing masks per instance either:


(left side master, right side instance, both showing the physical material mask; can confirm this in gameplay as well, so not just a visualizer quirk)

Of course, I could create a unique master material for every single composite asset, then the masks would work, but that’s insane, so: What do I do? Am I using physical material masks right? Is there an easier way to do this?

Working in 5.5