Okay, so there are multiple places where things could be busting: I've messed around with the alpha gain and offset in the color balance tab of the file node and although I can adjust the glossiness of the skull to the right value, the teeth don't have a different value, so it's like the engine isn't reading the files at all. I've switched between RAW and SRGB in the file's color management, I've turned on 'alpha is luminance', and nothing works. I then tried deleting the shader and reconnecting everything, but this time the whole skull is glossy, as if it's reading the maps as pure black. The lower jaw's UVs are in the 3rd UDIM, so I checked the UV editor and all 3 UDIMs were loaded into their correct place, but it was as if the engine wasn't reading the file. I kept working on the lighting and such, and then seemingly for no reason, the lower jaw of the skull started rendering absolutely glossy. When I imported the new roughness map into the Arnold shader, it was working pretty well. Here's the skull with the roughness channel on, and here's what it looks like inside Substance painter. The base for the map was the previous one, except with the added details.

Yesterday I decided to give the roughness map some more detail and manually paint roughness into the small cracks and pores of the Skull, as well as occluded areas which would be dirtier.

Here's the result render in Arnold at that point. Afterwards I made a simple roughness map in Substance Painter which simply made the teeth slightly glossier. I then baked normals, displacement and texture maps. I made a skull in Zbrush, then did the retopology in Maya with 3 UDIMs.

Substance painter arnold