avahi-browse -kprtf _apt_proxy._tcp does not show any output, but the server is actually alive

[Norbert]

Steps to reproduce:
1. Have Ubuntu 20.04 LTS installed
2. Have `squid-deb-proxy` on the same local network
3. Install `squid-deb-proxy-client` package on 20.04 LTS
4. Monitor logs of `squid-deb-proxy` server with tail
5. Run any `apt-get` `install`/`download`/`upgrade`/`update` on 20.04 LTS client system with

Expected results:
* 20.04 LTS client system uses `squid-deb-proxy` server via `squid-deb-proxy-client` package

Actual results:
* the avahi-browse does not work, so packages are downloaded directly from Ubuntu mirrors instead of `squid-deb-proxy`

Debug:

* the command `avahi-browse -kprtf _apt_proxy._tcp` which is called by `/usr/share/squid-deb-proxy-client/apt-avahi-discover` does not return any output on 20.04 LTS client. Other clients - 16.04 LTS and 19.10 have normal command output for this command.

Notes:

* avahi-discover GUI window is empty too.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: avahi-utils 0.7-4ubuntu6
ProcVersionSignature: Ubuntu 5.4.0-18.22-generic 5.4.24
Uname: Linux 5.4.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu21
Architecture: amd64
CurrentDesktop: MATE
Date: Fri Mar 27 19:25:45 2020
InstallationDate: Installed on 2020-03-25 (2 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Alpha amd64 (20200325)
SourcePackage: avahi
UpgradeStatus: No upgrade log present (probably fresh install)