Thanks!
We retouched the scale and collisions of the objects for the polished version.
Yes - to prevent items dropping off the belt, rails probably need a 1:10 (Very thin wall) kind of invisible collision on each side, then are given layers which only interacts with items that are not yet dragged with mouse. As items would still need to cross the invisible collision if player picks them up. Sadly, it seems to be too much for polishing phase. But, definitely something we are made aware.