Thank you for sending the sample publication.
The behavior that you described as working in earlier versions was actually a side effect of a bug. Before version 5.7.1 snapped objects from the master page and the current page did not align correctly. They should have aligned as if they were on the same page which is what now happens in v5.7.1.
In looking at your sample pub in version 5.7.0, I see that the image on page 1 covers the objects on the master page. To create the same effect in version 5.7.1, you just need to hide the master page items for that page - as you already discovered. If you need to control the visibility of the master page items programmatically, you could instead use ShowObject/HideObject to show/hide the container objects on the master page. To have the image appear behind the master page objects, you could set the image as the page's wallpaper background - see Page Properties in the Page menu.