libgdx – Transparency of the imported Blender object related to the color channel and not to the alpha channel

I created a template in Blender 2.8, set some materials to different shades of gray and exported to fbx format.
I then used the fbx converter to get a g3db.
I opened it in a libgdx game and all the textures are half transparent …

Ideas to solve this problem and what is the cause?

Breeding steps:

Mixer 2.8
Libgdx 1.9.9

Hardware configuration

Export as fbx (default, scale 0.01)
fbx-conv-lin64 -f file.fbx