Sorry for the delay. The issue you're running into has to do with the image processor we're using. Until we come up with a solid solution, there's a couple ways to work around this.
The problem occurs when there's an alpha 0 pixel in the image. The processor doesn't know how to handle it properly, so it treats the image as a grayscale image.
You can trick the system by putting one pixel of color in the image. This will make it so the processor is forced to treat the image as a color image and all of your alpha 0 areas will remain.
The color pixel doesn't have to be bright. You can just shift the main color of your image on point to red, blue or green and the workaround will work.
It's not the most elegant solution, but it does work for now and gets you moving on to more important things.