I've taken a quick look at this. I can't see any problems. I see the ball drop to the ground and stop, that's all. What are you expecting to happen? I'm not Davo, and Davo wrote the Classic custom movement, so none of this is obvious to me I'm afraid, you're going to have to spell out what you're expecting to happen.
I see the ball drop and then onGround is true and the Custom Movement's dx and dy are both 0 (because it has stopped). I don't know what Davo did in Classic, but in the HTML5 custom movement, if dx and dy are both 0, it doesn't step, because there's nowhere to step to. So the first event will not be running (On CustomMovement Step). Perhaps the Classic custom movement still stepped there?
* There's no 'decelerate' action for the Custom Movement. Why there is none is beyond me, honestly, it's practically necessary for any movement that needs the object to slow down gradually rather than stop on a dime.
Isn't 'decelerate' the same as using a negative value for 'accelerate'?