I was tinkering with the custom style for my Garmin Edge 800 recently and noticed some of my lines were looking bad. The color for the background object was showing through for parts of the line, and the line would be missing completely where it crossed water.
It turned out the problem was the combination of having the line style set to a bitmap while having the orientation flag enabled. Disabling useorientation eliminates the partial transparency.
I also saw that roads appear much smoother if you define them using border + line width instead of using a bitmap. So it is best to avoid using a bitmap for a line if you don’t need to create some sort of pattern. For the black and yellow line at the top, I increased the border width to 2 and reduced line width to 1 to avoid using a bitmap.
Maybe this isn’t an issue on newer units.