Attached a video demonstrating the symptom, and pictures of the setup. Video starts just before the Drain Cycle ends. Once it ends, air slowly enters from the disposal-end, pushing all the water from the top of the high loop back down into the dishwasher. It rests near the bottom, where the water level ends up inside the dishwasher. Shouldn't this not happen?
so some of the waste water from the dishwasher returns to the dishwasher when it finished pumping that's completely normal. There's no real way to prevent that.
So long as you don't get water from the sink or from the drain flowing into the dishwasher everything is fine.
This is exactly what an air gap is supposed to prevent. This is something that sits on the edge of the sink, and lies between the dishwasher and disposal, but I don't see one in your setup. This ensures that any backflow pressure won't go all the way back to the dishwasher. You can buy one at any home improvement store, but installing it might be annoying.