Thanks for the help - it turns out I was drawing the same gradient image multiple time in a loop (there is a small animation involved), this made semi-transparent pixel look completely opaque.
My loop was essentially doing something like
[start loop]
canvas.clearLayer(1)
canvas.setLayer(1, myContent)
[end loop]
..apparently this wasn't enough to clear the existing layers drawn on each iteration, canvas.show() needs to be called in order for the clearLayer() command to take effect.
Quick rant: the roImageCanvas apis are severely under-documented considering they behave in ways that are less than intuitive. In particular I have a number of unanswered questions related to multiple layers (they don't seem to be drawn independently of each other, aka clearing a #1 layer will also clear a #2 layer). Of course I could build a dozen of test cases to figure out answers one by one but why make it so hard/time-consuming on developers
