I fear that is the way the hoppers are designed:
The creator has to create the illusion that coal shows up when loading the hoppers. To do so, a visualization of the coal has to be available somewhere in the asset. Some do so by flipping the image / mesh representing the coal around so it doesn't show in the car when empty, others (like in this case) do so by putting the image / mesh way below the car... which usually is far underground where nobody will see it... unless driving over a bridge higher then the dept the coal was hidden.
I suggest trying a few different hoppers (one not made by the author of this hopper) and see if the effect is also there.