Using net-print/cups-2.2.7, it depends on the browser used if one is asked to enter a username and password when doing some administration via http://127.0.0.1:631/. With Falkon, Chrome and Opera, one only gets a "Not authenticated" error page. Firefox does still ask for a username and password.
This seems to be a known upstream bug: https://github.com/apple/cups/issues/5289 sorry for the noise ...