Funny bug, that I used to my advantage:
- I have a truck at the mine, on the unload zone. It displays: press button "X" for unloading;
- I switch to another truck, anywhere else. The same message is still displayed (even on the plain road, or in the entrance of the garage or a trailer store, which I thus cannot use);
- I press "X" and I have the menu for unloading at the mine, and it works;
- don't know if it's related, but even after unloading everything, the mission is not completed;
Also had this for the conveyor missions, it is probably related to all the new unload zones.