[MrGoodtunes]

Not willing to try flashing back or anything so drastic, I sort of tabled this annoyance awhile. Then, just the other day, I remembered about a different way to adjust backlight sleep settings. Instead of directly changing registry value(s), and instead of going through backlight settings, I tried going through battery settings. And it worked!

Try this: Tap the battery icon near top right of today screen. Next, tap bottom left softkey Menu, and choose Backlight. It looks as if it's just a different shortcut to the same exact thing that's been failing, but for whatever reason, this way works for me. Maybe because of something else?