This was also brought up in the other thread but I guess I'll post again since you have a .cap that can demonstrate this..
If you change one of the moving platform's sinewave behavior to horizontal you'll notice some other problems as well.
1) If you're being pushed to the left you will have a hard time jumping (I guess you get stuck in the platform a bit)
2) If you're being pushed to the right, your player will 'hop'. If you hold left while being pushed to the right, the player sometimes 'climbs' over the platform.
This has nothing to do with the collision mask on the sprite - the same thing happens in my game with a rectangular collision mask. Also, this happens with any moving platform, not just ones using the sinewave behavior.