It has to be something with the principled. Bottom line is the maps work in Poser versions existing for years, including variations of Superfly. The fact that standard png file textures offered free with no restrictions might not work in an obscure render material feature in a version of Poser that's existed for months and is apparently full of bugs, brings virtually no new features to the table, handicapped on the script front, and is still in beta is of negligible concern to me. I'm using a version that works, with simple texture files that work. Apologies they aren't fitting your apparently very very narrow criteria of "working" files. Not to come off as a harsh but those are the facts.

Take care