When creating a pack, A PBR texture cannot be used inside of the texture variants or it loses it PBR properties. This has nothing to do with multiple pack or inter operability as the original ticket was closed upon. In 1 pack you can in the block definitions json file refer to the same target textures for 2 blocks. If you define a block json file. And you use variants the Physical Based Rendering objects are dropped. If you do not use the variant settings the Physcial based rendering files are used by the game engine
So TLDR, When using the variants settings, even if all of the variants have PBR files, the minecraft RTX engine ignores them, even if only 1 variant is defined. If you do not use variants the PBR textures work as intended.