Thanks, I already got the originals from the SketchFab site yesterday.
Yes, I'm still seeing that glitch in my latest OR. But it reveals itself only in the legacy FFP mode. PPL renders the model all right with the FBOs either on or off.
I'll look into what might go wrong with FFP when it meets that particular combination of the model's raw materials/textures. FFP is emulated on modern GPUs with a built-in shader that might conflict with the user shaders resident on the GPU when it toggles between GLSL and immediate modes of operation. We had a similar problem when emulating your "ambient reflection" stuff in the Blinn-Phong (no-bumpmap) pipeline.
While we are at it, I think we should implement that map_d alpha mask approach in OR too. Using separate 24-bpp diffuse and alpha masks is a classic OBJ format mode of operation. I think it won't be too difficult: whenever OR comes across a separate map_d alpha mask, it should add it as an alpha channel into the existing normally opaque 32-bpp diffuse map.