just noticed on your screenshot it has something mine does not: under meshes window "Type: Standard/Terrain." Not present for me in the program.
Viewing post in 3D Model Converter for GameMaker comments
The type at the bottom is something that’s in the development version that I don’t include in the “official” version (yet), it’s related to this. It doesn’t have an effect on how files are loaded.
I think I’ve fixed it, if you go into Preferences and make sure Combine OBJ Vertex Colors and Combine OBJ Submeshes are turned off that’ll prevent the program from overwriting the vertex color with the material color when you import files. I’ve also uploaded an updated version where vertex colors are blended with material colors instead of overwritten by them. Hopefully that’s the end of the problems.