https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=194583946 if you place an object (more specifically boxes) inside the back of an A3 truck, and the object moves inside of the back it will bring the left and right sides of the back with it, so that the object eventually falls out. Like this: https://steamuserimages-a.akamaihd.net/ugc/4037121625253194/E64E8C235A8DD86FB49BEC1A63667A7699F7C516/?imw=5000&imh=5000&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Or this: https://steamuserimages-a.akamaihd.net/ugc/4037121625252849/05293C1090A11F2079F8D17F2C177CD2DE4611F5/?imw=5000&imh=5000&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Im on the x64 branch, but it also happens on the Default branch.