Why doesn't the screen of my Garmin Etrex 20 go off as expected?

Even after setting "Backlight Timeout" to 15 seconds in Setup > Display, for some reason, the screen of my Garmin Etrex 20 doesn't go off.

FWIW, its software was upgraded recently to the latest and greatest 4.20.

Any idea what it could be?

Thank you.