I’ve found the solution.
Apparently it is necessary to save out the textures before exporting as .glb/gltf otherwise you end up with this glitch. This also happens when you tell the .glb/gltf exporter to export your images as ‘JPEG Format’, so don’t do that.
I don’t know if this is a bug or something like that because it doesn’t happen with .fbx format.
Anyways saving out your textures before exporting will solve this issue.