Before FW 8.20 (outside of an activity) the backlight would stay on as long as you kept pressing buttons within the 5sec timeout. This makes sense; the backlight exists so you can see the screen in the dark, and if I'm still pressing buttons it means I'm still using the watch.
However with FW 8.20 the backlight times out after 5sec even if you're still using the watch.
Unless the purpose of having a backlight has been redefined by Garmin (most people would agree it's to see the screen in the dark while using it) I assume this new behavior is an inadvertent bug.
Can one of the mods please pass this concern along to the Garmin to fix ASAP. It's very annoying if you use your watch at night a lot (having to keep pressing the light every 5sec).