When the “Fixed Header” design option is enabled in Genesis DevKit you can preview the various devices to see what they will look like to your site visitors. However when set to “All Devices” your site will then display a fixed header on mobile devices as well. In this case it is import to note that though non-logged in visitors will be able to view a fixed header on a mobile screen/device size, you will not be able to do so when logged into your WordPress Dashboard. This means that when you preview your site in “Mobile” view it will show as a non-fixed header, scrolling up the screen with the rest of your site content.
The reason for this is to account for the fact that at 600 pixels and below the WordPress admin menu no longer fixes itself to the top of your site, but scrolls up as well. So when this happens, if you mobile header were to be fixed at that point, your site would have a large empty space between the top of your site and the top of your header.
So this is all just to say that your site visitors will get the mobile fixed header experience as long as they are not logged into your WP Dashboard, but you just won’t see it in the Mobile preview window in your DevKit admin pages.