I can see where this would be useful if you have another line stopping at the same station that you want to transport those goods. The problem in TF is that, unlike TTD, the player currently does not control how cargo is routed. As a player you’re just making lines available and letting the AI determine how they are used.
In your case the AI has determined that loading up those goods is a more attractive option than whatever else you wanted to happen. The fact that there’s only one car type for multiple goods also doesn’t help as you can’t even use the consist to define what should be carried.
This seems to be the design philosophy underlying the game, so I don’t know if it’s going to change. The only thing you can currently do is make another station and route somehow more attractive to the AI for the goods.