I came on here because I had this same problem but I kinda figured out where the issue lies.
The problem is that they binded the drone descend key to "Crouch Toggle" in the Infantry/Passenger Keybinds; For those that don't use Toggle like myself and only use HOLD crouch; that key is not binded to the Drone so it will show up as empty. They also binded Drone going up to Jump; which is spacebar for many people; I agree the drone needs it's own key bindings to fix this or have them match up with the helicopter Ascend or Descend keybinds.