Hello,
I have the same issue, with different renderers using alpha map and assigning alpha channel as RGB output works fine, however you have to use a black and white image (not supposed to be 8bit) in Corona version Alpha 0.7. I am no good at programming but is it possible that this might be a 32bit image issue, while a RGB image is 24 bit, an image with Alpha is 32 Bit. I'll try gray scale image with a different Alpha and will inform you.