Why does the Back button minimize a UWP app?

Question

.NET

I was developing my app for UWP, and today when I launched the app, I found that pressing the back button on the phone or emulator simply minimizes the app. It used to take me back to the page I navigated to using the Frame.Navigate method, but now it just collapses. Only this construction helped: https://social.msdn.microsoft.com/Forums/sqlserver... But even it works crookedly, it throws one more page back. Can anyone explain what happened?
ZY Everything works in my other projects.
Code of these two pages pastebin.com/SsVXAVsc