I made a very minimal pluging 

namespace Oxide.Plugins {
  [Info("Test", "Nurdism", "1.1.0")]
  class Test : CovalencePlugin {
    private void OnCargoPlaneSignaled(CargoPlane cargoPlane, SupplySignal supplySignal) {
      Puts("OnCargoPlaneSignaled works!");
    }
  }
}

and this event documented here: https://umod.org/documentation/games/rust#oncargoplanesignaled doesn't seem to fire. Its causing a few of my other plugins to not work correctly. 

These are the versions I'm working with:

From testing:

You're right, it was another plugin that was taking over signals