You have a work-around for this issue.
1. Add a Screen named: "Screen Saver"
2. In the "Screen Saver" screen add a "Fixed Text" element, without text, and a Black background, and "Fit it to screen" (right click on the element -> Fit to screen).
3. Add to this "Fixed Text" a Action "Load/Last Screen".
4. Add a Global "Load Screen" Action to the project, bind it to a bit named: "Load Screen Saver Bit".
5. On the IsActive of that screen, add a global action that sets the back-light to 10%, and when existing that screen, you can make the back-light back to 100%
Now you can set this bit ("Load Screen Saver Bit") anywhere you want (Ladder, HMI, Timer).
As for the 0% in the UniApps, setting the value to 0% actually doesn't turn it off.
We will add this feature as a feature request, but I don't know if, and when it will be implemented.