Some FBX files contain somewhat incorrect relative paths for their material textures. The old Python importer was "working" only as a side effect that it was exhaustively searching the subfolder hierarchy. Try to do a smaller version of that here too: if referenced image is still not found after trying the regular relative/absolute paths, try taking longer path parent chains from the absolute path coming from FBX file, and using that as relative path wrt the FBX file itself. Pull Request: https://projects.blender.org/blender/blender/pulls/145641