Update 2. I found the issue. for the missing displacement thingy.
It appears that, if the displacement is populated in a material function that has a MakeMaterial node output, the displacement is not propagated to the Main material and it’s all black.
I propagated the displacement map in the metallic then remapped metallic to displacement in the main material, and it worked. Is it supposed to work this way? Is that expected? I hate the workaround, but it seems to be the only way to get this working this way for now.