Your lock's acting as designed; it's intended to work the entire central locking system (doors, gascap, trunk).
However, if you turn the key 90 degrees (I believe clockwise)and take the key out, the trunk will stay locked even if you unlock the doors (from the door key). A valet feature to keep the trunk secure as I understand it. The actual valet key won't work on the trunk lock nor the glovebox, so if you've properly locked the trunk it's secure.
To lock the trunk at the trunk, turn 1/8 turn clockwise; that will lock everything. If it requires a lot of pressure, then something in the mechanism needs lubrication but I cannot help on that point
All the best, Michael