I think more than a screen shot might be needed, perhaps a .glb. When I recently got a grey material when I was expecting a PBR texture, this was due to an error in the .babylon exporter alpha for Blender 2.80. I forgot to switch to generation of a diffuseTexture to an albedoTexture.
One would think that this released exporter would not be setting diffuseColor instead of albedoColor, but that certainly looks like the effect.
I really cannot answer this directly. Since your material is very simple, you might just replace the node with a Principled one with the same settings, hit the ‘UsePBR’ check box, and see what the .babylon file version gives. The .babylon exporter might be using the glTF’s nodes in the future, but not in the current test. It also only runs in 2.79 today. If you have issues, please post the .log file.
Does ThreeJS have a sandbox, or some other easy resource out there, so it can be isolated whether the problem is on the exporter, or BJS side? Since you are using the sandbox & not using any textures, it would not seem like it was your error, for now.