Windows 10 Screen Saver quit working

Under Windows 10, you have two methods of making your screen go dark after a set interval of inactivity. One, under Power Options, actually seems to turn the LCD display backlight off after the programmed interval. The other is the Screen Saver, and when 'Blank' is selected as the pattern, the screen goes black, but the display backlight is still on.

I've been using the Screen Saver for years, and it has always turned the display black, even with applications active and running. But this has suddenly quit working. I checked the settings, and they are right, but the screen stays lit. I tried the "Turn off the display: 10 minutes" under Power Options and it does work as intended, so at least there's a fallback position. But I wonder why the Screen Saver quit. Any ideas?