PBR material definition


I am wondering what is going wrong when using the PBR material definition and using a texture where the RGB channels are packed with the AO-Roughness-Metallic maps.

The Khronos group says to do this,

The Ambient Occlusion, Roughness, and Metallic textures are saved in a single channel-packed texture to reduce the number of texture loads. The textures need to be packed into the channels of your texture as follows:
•Red: Ambient Occlusion
•Green: Roughness
•Blue: Metallic

the Khronos group Blender node setup is as follows,

it sets Linear space for the non-color texture metallic-roughness and the AO sockets.

I see in the PBR definition this,

I get this warning about the setting the LightMap when doing this.

The texture xxx.png has linear color space, but the material parameter LightMap specifies no color space requirement, this may lead to unexpected behavior.
Check if the image was not set to another material parameter with a linear color space, or that you did not set the ColorSpace to Linear using texture.getImage.setColorSpace().

How can I fix this or is this a bug in the PBR material?