Windows 10 is the latest operating system provided by Microsoft and now it has almost 35% of the Windows computers market out there so the Sign-in method with PIN or Password is a nice add on in Windows 10 .
So when you sign-in with your user account or unlock you computer, the last used sign-in method (PIN, Password or Windows Hello) is saved to the registry, and the same will be used the next time, but the default sign-in option reverts to password at every time you restart your computer, even you used another sign-in method to login last time.
One of the reasons why this problem occur is you automatic login using username and password.
If you have automatic login enabled, your account login automatically and immediately updates the last used sign-in method or the authentication mechanism, in the registry.
So if you want to set up PIN as the default sign-in method disable the automatic login option.
To disable the automatic login option, click(tap) on start button and in search type netplwiz.exe or userpasswords2 . Enable the check box “Users must enter a user name and password to use this computer”, and click OK. This will clear the saved auto-logon credentials.
Speaking of alternative solutions, if you want to keep your PIN as the authentication method but still need the autologon feature too, you can check out LogonExpert. It supports native PIN autologon to Windows 10 and also keeps the logon credentials AES-256 encrypted.
Hope this works, good luck !