This scene should have three materials, but only one is visible in 3D settings.
Models are exported as OBJs, but the problem persists with other file types as well. The matching MTL files confirm that meshes are using three distinctly named materials.
An ugly work around is to segregate models and export in another file format. I.e. scenes with only OBJs have only one material, scenes with OBJ + FBX have two materials, scenes with OBJ + FBX + DAE have three materials, etc.
Here’s a version of the scene exported using three distinct formats:
I’m assuming it’s a bug and not functioning as intended.