Thanks for providing that scene file GlaDOSik! It made me realise that I hadn't saved the material as a separate .mtl file. That seems to be what made the difference.
If you use a standard fixed material , it allows you to use glow without saving it as a separate .mtl, whereas when you use a shader material the glow will only work if you first save the .mtl. Doesn't matter if the shader is "material shader" or "material shader graph".
Anyway, it is now working and saving an .mtl file for each object would be good workflow practice I will use from now on. Especially considering every other item in Godot(script, mesh, scene, etc) is saved as distinct asset.
This could be classed as a bug, so I will do a bit more investigation and file a guthub report if it warrants it.