Items will shift through the storage walls.
This seems to happen at all upgrade stages of the carriage box on the tank. This is easiest to reproduce with Springs. They will fall on the ground when the tank is moving if there are other items in the back. When a Metal Sheet falls on the Spring it will phase through your storage box onto the ground. Can happen if there are only Springs in the back aswell. Metal Sheets will also phase through the box; I had 4 Sheets in the back and they were not nicely arranged and they flew off when moving on flat ground. Placing them flat on the bottom made this not happen.
Aside from these two items doing this while simply moving, most non-cubic items will do this on impact. Whenever I drive over an item on the ground the tank crashes and items try to fly away (which is normal) but some manage to escape from the sides, not from the top opening.
I can do more testing if required.