Cups detects only unusable printers with KDE apps, while firefox finds usable printers.

Issue description

Until november (maybe), I was able to print without any problem on my KDE Plasma unstable nixos in my lab (ok, sometimes I needed to restart cups_browsed... but otherwise it was working). However, since november I have a very strange problem: cups detects only "bad" printers, and I cannot print on them. Indeed, all the printers I can find in http://localhost:631/ appears with a bad url: Connection: file:///dev/null and the status is either paused or idle. However, if I go to http://cups.mylab.com:631 the printers that appears are the good ones and have good urls like lpd://location_printer.common.mylab.com/raw.

Now, let's continue with another fun fact: from the "Printer" section of KDE, or also in libre office, okular, and as far as I can see all KDE based applications, the printer displayed are the bad ones from http://localhost:631/. However, on other applications like Firefox, chromium, gimp, both the printers from http://localhost:631/ and http://cups.mylab.com:631 appears. The bad printers are grayed and not usable, but I can print from the ones that corresponds to http://cups.mylab.com:631 successfully.

From the logs of cups, I don't really see anything valuable, but maybe you can find some information:log_cups.txt

Steps to reproduce

In my nixos configuration you can find:

services.xserver.desktopManager.plasma5.enable = true;
# Enable CUPS to print documents.
services.printing.enable = true;
services.printing.drivers = [ pkgs.hplip pkgs.hplipWithPlugin pkgs.gutenprint ];
# In order to be able to detect printers
services.avahi.enable = true;
services.avahi.nssmdns = true;

I am not an expert of cups, so I can't tell you how the network of printers is setup... But once it's setup like mine, try to go to the KDE system of printers, or okular: you will see only printers IDLE or paused with url pointing to /dev/null. On firefox, chromium, gimp, you will see both working and bad printers, but the bad ones will be grayed and you will be able to print.

Technical details

system: "x86_64-linux"
host os: Linux 4.14.102, NixOS, 19.03pre170287.19eedaf867d (Koi)
multi-user?: no
sandbox: yes
version: nix-env (Nix) 2.2
channels(leo): ""
channels(root): "nixos-19.03pre170287.19eedaf867d"
nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

NixOS / nixpkgs