Im looking at it, weird bug but prob fixable, first of all try not to use that many collision polygons, since it takes a toll on performance. Try use for example for the ball, use circle poly, and for the player use a square poly in the drop down list.
I have never done a platform with this much collision checks, usually i do "tilemaps" like 1 square for a hill then 1 square for a flat ground, and i snap them together with snap to grid mode.
Im checking at your project now if i find out why its doing this i will tell you :)
EDIT: I have checked, i am at work so i have not found any good solution yet, nothing in console either, when i get home i will try to find the glitch, but i still suggest you use less collision polygons, i mean your game is running smooth now doesnt really matter, but if your doing multiple levels later on etc, would be good to think about performance.