Well... I do not use cups, so I have no idea about its web interface.
But I do use transmission-daemon that has a credentials protected web interface, and that one works fine, on all my browsers. It is not on https.
Firefox
2019-10-17-173834_1280x1024_scrot.png
Chromium
2019-10-17-173714_1280x1024_scrot.png
Opera
2019-10-17-173732_1280x1024_scrot.png