winstreak Thanks so much. Assigning the Destination Out blend mode to the mask and making the mask the top-most object on the layer was the solution. I went through all the blend modes a thousand times, not sure why I didn't stumble upon that result. Thank you.
FWIW, I only need to use a single mask object in my case, since the top edge of the slider thumb object is flat (not rounded). Tracking touch to change the height of the thumb works well.