Control: tags -1 + moreinfo https://buildd.debian.org/status/package.php?p=avahi&suite=buster looks good, i.e. avahi has been built on all architectures successfully.
Could you paste the output of apt-cache policy libavahi-client3:amd64 apt-cache policy libavahi-client3:i386 Maybe you have a package put on hold or some mirror is not up-to-date yet. Michael