I had the exact same issue with the measurement units (miles / KMs).
These were re-setting back to miles, even though I had these set to KMs.
I was selecting the unit on the watch from the setting menu.
Every time I was syncing (i.e. connecting to garmin connect) it was re-setting.
So ... make sure that you set the backlight option not only on the watch but in the application as well. SEtting the option in both places will, I think, resolve the issue.
If you have the Back-light setting on the watch to activate on Gesture "only during activity it reverts to "activate on gesture" after each sync.
I had the same problems with setting the units (miles / kms).
I had selected KMs, but after each sync I was reverting to miles.
The reason was that I had the selection done on the watch (through the settings menu). But the Garmin COnnect user settings I had not made any changes.
Try setting the backlight option both on the watch as well in Garmin COnnect.
