I have found that in Windows 8 apps, a Page's constructor always gets called again when navigating back to this page, however this is not the case in Windows Phone 7 apps.
Because of this behavior all the properties of that page get lost and you need to somehow store them and reassign them after the page is reactivated. At the moment I am using the SaveState method to save the data and reassign the data in LoadState method. Both of the methods are built-in in the sample Grid/Split App.
I wonder why it's designed this way and if there's any better way to maintain the instance of the page?
You can set Page.NavigationCacheMode = NavigationCacheMode.Required for each page you want to keep in memory, but note that you have to make sure users can't navigate through these pages endlessly as that would leak memory.