Thanks for your feedback! Yeah, the cube probably fell off, if you send it through too fast it will be launched out and it can fall, it was supposed to be some sort of intended mechanic to "force" you to realize you messed up, lost the cube, and go back to the red button where the cube was to recover it.
So, TLDR, cubes have a small red button at their starting location that will retrieve them, that's how you get it back.