Having had this issue on multiple occasions and resolving it by manually editing the MTL file with `map_Kd texture.png`. The cause ended up being that creating a Shader Node in Blender 2.8 material with `Image Texture > Material Output` wasn't enough for Blender to parse the flow graph for the texture details, and it was necessary to create the material like this: `Image Texture > Principled BSDF > Material Output`.