When you create a new page and select a template (other than the top most) from a list of many templates under your _Templates folder, everything seems fine until you try to edit the page.
What happens is that it fetches all the correct data/elements from the selected template and it is visible in Edit Mode in Content Studio on the Edit/preview page. However, when you select Customize page it fetches the Components from the top most template in your _Templates folder and shows them in the components list on the left side.
Which means you are shown the elements on the page from the correct template but the components-list on the left side does not match and you are not able to edit any of the elements on the page. Which render all templates, except from the top one, not usable.
I have tested this in 5.1 and 5.1.1 of Content Studio and both show the same faulty behaviour.
This works as expected in Content Studio 5.0.5.