Gimp Normal map affects COMP green channel?

[Douglas]

I am creating a livery for an Aeroplane Heaven model for MSFS. I understand PBR, and I am able to get the look I want using the amazing tools in Gimp which is my main program for liveries.
I wanted to redo the lines and rivets on this particular model as they were quite messy, my repaint is depicting the restored aircraft so I wanted a better organized NORMAL. I redrew the entire lines and rivet network for the model using layers in Gimp 2.10. I started to use NORMAL Map feature ( Filters/Generic/Norm) and I got great results, but one thing has me baffled. It seems the new NORM texture is affecting the "shine" of the model in game (typically controlled by the COMP's Green channel). I don't understand how that is possible when I am working on the NORM texture exclusively. 
With the export, I made sure the Alpha channel was turned off, 16bit NORMAL, Y not flipped (the result of flipping the Y was not correct). When applied, the model lines, rivets, hatches, all look fantastic, except now the sheen on the model is extreme. Reverting back to the messy default NORMAL, sheen is correct.
Could anyone please help to explain how the NORMAL map generated by Gimp results in an over-polished sheen on the model in game?

Workflow and results:
Multi-Layered Master
Collapsed_Layers_No Alpha

Converted to NORM Alpha Off

Export to DDS settings
Shine difference  shine difference
Shine Difference Underside shine difference
Thanks for reading, any advice is appreciated.

Doug