The system, under the hood, works lik this: all negative numbers are allowed to disable a schedule. The gui just makes the current value negative.
That isn’t ideal on the node-red side of things, as I define all possible values in the service.json file and prefer to somewhat limit the size of that file. Hence the fact that I only define -1 there.
So at can indeed happen that a schedule that has been disabled by the gui can output -8 (undefined). Not ideal, but right now that is what it is.
For the next updates for the node-red-contrib-victron package I am planning on making some adjustments to the way things get defined in the services.json file and better cope with this.