Microsoft introduced a new way of managing default printer in Windows 10 build 1056, called Let Windows manage my default printer. Basically, when turned on, Windows will automatically make the last used printer as your default printer. The intention is to help ensure the best printer is pre-selected in print dialog box.
While it’s a good news to those who use Quick Print which always print content to the default printer, it could be quite annoyed to those who occasionally print to a different printer.
To turn this behavior on and off, go to Settings → Devices → Printers & scanners, and slide the option “Let Windows manage my default printer” on or off. The option is located right after the list of printers you have installed on your computer.
Because of this new feature, the ability to set a default printer by network location has been removed by now.
Note that the option is on by default. So if you are unaware of this feature but see your default printer being changed back and forth, this is the setting you can make things straight again.