If I am in the middle of a Bike activity and turn off my Varia backlight while resting, when I turn on the light again the Forerunner 935 doesn't automatically reconnect to the radar. The Edge 820 does reconnect as expected.
It seems I have to stop the Bike activity on the Forerunner and then start another activity, and maybe it would work with a 'Resume Later', although I haven't tried that.
I think the Forerunner 935 ought to reconnect even in the middle of an activity, just as the Edge does.