Thanks for making that example.
It reproduced on my env(win 7, mg 3.7). I think 10th generated image had white bottom.
However the problem went away when I changed backend from MonoGame.WindowsDX to MonoGame.DesktopGL.
So maybe problem is in png writing library(as I said before, these backends use different libraries for that task).
So the solution would be to either switch to MonoGame.DesktopGL. Or to use external library to write pngs.
Such as https://github.com/StbSharp/StbImageWriteSharp that is used in the MonoGame.DesktopGL
The code to save a texture would look like following:
// Saving texture in SurfaceFormat.Color
var data = new byte[texture.Width * texture.Height * 4];
using (Stream stream = File.OpenWrite(path))
ImageWriter writer = new ImageWriter();
writer.WritePng(data, width, height, ColorComponents.RedGreenBlueAlpha, stream);