Surface Pen now writing funky

[Ofnuts]

Possible causes:

• "Opacity" slider of the tool not set to 100%
  
• Using some "Dynamics" that interacts with opacity (typically some Pressure/Speed > Opacity setting)
  
• Painting on a layer whose "Opacity" slider is not 100%
  
• Image in color-indexed mode, and no true black on the colormap, so Gimp uses what is closest.