I encountered the same problems when I developed my paint kit and skins for Manfreds C-47 and C-117 V2 models. The display conflict is in the bump map alpha channels.
I would suggest that painters compose their bump maps using the early methodology which addresses certain FSX shader anomalies.
To compose your bump map, first create your grayscale image to suit your artistic requirements.
Create to bump map using the Nvidia plug in or your preferred technique.
From that point, create a new alpha channel in the bump map file. Copy the red channel into the new channel. Next, fill the red channel with black, invert the green channel (not turn upside down but use Image/Adjustments/Invert or Ctrl+I), then fill the blue channel with white. Your finished bump map should be blue.
Compile your various textures into the aircraft file using the usual methods.
This should resolve the two tone shine issue in Manfreds DAK and Skytrain model(s).