Why does transparency disable subpixel rendering in Adobe Reader?

Subpixel rendering is done by using the individual colors of a pixel. Every pixel on a screen is made up by an even smaller pixel with the color of red, green and blue.

If you want to render an object that is 10.3 pixels wide, you would use only the red color of the 11th pixel. The screen would then emit light from 31 subpixels, that is 10 full pixels and then a third of a pixel.

If you try to simulate this effect yourself, simply by drawing 10 white pixels wide, and then a red pixel on the right at a black background, you would see the effect yourself.

The problem is that you would notice that the right edge would be red, this if you simply do it that way. Most likely, Adobe has to use advanced algorithms to cancel out that effect - possibly by amplifying the value of the red and green of the 10th pixel.

Adding transparency to an algorithm that is already quite advanced is probably something they haven't solved yet and therefore, they simply disable subpixel rendering.

I believe this to be the case, because I've tried to implement subpixel rendering myself in this naive way and I saw that "coloured side" effect myself.