@noersaleh All Right!.. Preliminary tests with a clean Project point out what I guess is obvious, after the fact 
… but not documented that I am aware of (as far as the API is concerned).
If you don’t have a widget assigned to a digital pin, it will register as “Requested pin doesn’t exist in the app.”… or possibly a random, unchangeable, state.
But if you then assign a widget/reading rate to a digital pin, then the API GET command will read the corresponding pin state [“0”] or [“1”].
The key is that you MUST have an pin assigned widget!.. vPin for Vpin reading, GPIO assigned for Dpin (or Apin) reading.
Toggling a digital pin however WILL work without a widget assigned to the pin… at least with the Write pin value via GET method… go figure.