I met a couple people in Iceland last summer that had been backpacking and bought some inexpensive folding bikes to continue their trip. They planned to just strap a big backpack on the back. I thought it looked hilarious, had to take a few photos. I gave one of them a strap that I was not using since they clearly had problems getting their packs to stay exactly where they wanted them. I have no idea if they rode their bikes out of the campground later that way or not, and if so I have no idea how they did. But it certainly looked unstable to me.

When I want to check a load on a bike, I stand to one side of the bike, grasp the saddle with one hand, the stem with the other hand, and robustly shake the bike from side to side to see how the load looks and to see if anything moves more than I want it to. If they had done that, I am sure their packs would have fallen off the bikes.