So it seems that OnCargoPlaneSignaled event doesnt fire for what ever reason, and that is causing the activeSupplySignals to remain blank when using a supply signal, that causes every event to be a "Timed Supply Event" so I have come up with a patch that fixes this issue: 

    private void OnExplosiveDropped(BasePlayer player, BaseEntity entity) => OnExplosiveThrown(player, entity);
    private void OnExplosiveThrown(BasePlayer player, BaseEntity entity) {
      NextTick(() => {
        if (!(entity is SupplySignal))
          return;

        var dropPosition = entity.transform.position;
        if (activeSupplySignals.ContainsKey(dropPosition)) {
          return;
        }

        activeSupplySignals.Add(dropPosition, timer.Once(900f, () => activeSupplySignals.Remove(dropPosition)));
        PrintDebug($"A supply signal is thrown at {dropPosition}");
      });
    }

This is only if another plugin is blocking `OnCargoPlaneSignaled` event, I found out that a loot plugin was changing how that SupplySignals worked and it blocked the OnCargoPlaneSignaled event

In the absence of the OnCargoPlaneSignaled hook, I use the way you do, but it is not very precise and I have to use the OnCargoPlaneSignaled hook in order to improve the precision.
For another plugin that blocks the OnCargoPlaneSignaled hook, I suggest having the plugin developer call this hook manually