Sync turns off under Android 6.01 Despite wifi set to 'on' when my two devices sleep  and power saving set to automatic (3 days of no use before the app is not allowed to run in the background)  sync turns off within an hour after both devices go to sleep. It can only be turned on by clearing the app out of memory and restarting it. Very annoying. It never did this under Android 5.x.
September 25 2016