It’s not equivalent to walking onto a moving sidewalk because you definitely have horizontal inertia when you step onto a moving walkway.
It’s not equivalent to stepping off the back of the truck.
I have had this situation happen to me almost exactly as described on a treadmill, and I can tell you what happens.
Once, while running on a treadmill, the belt had a small tear in it, and what happened was it caused the belt to fold in on itself and jam, bringing it to a stop very quickly and jamming it entirely.
What happened to me is that I lost my balance and almost fell over on my next step or two down, and had to grab onto the handlebars. It was very scary. I was running at 7mph I believe. Definitely no slower than 6mph. I heard the loud noise of grinding to a halt, lost my balance and freaked out. I would have fallen on my ass or face if I didn’t have something to grab onto.
Unfortunately my memory is not good enough to state for certain whether I would have fallen backward or forward, but I seem to recall falling forward, as if the rug had been pulled out from under me, leading me to feel like I was falling forward. But it happened in an instant and I caught myself, so I can’t be sure.