Hello @agibson!
This command line is a little different from yours (above). It separates each log by date. I don’t care about the message logs a year ago. How about including them since 1 Dec 2020 (or the first date in December)?
for logf in $(ls /var/log/messages* | sort -rV | tail -12) ; do ls -al $logf ; zgrep -ic "SERVFAIL" $logf ; done
Also what Core Update are you currently using? I saw Core Update 151 above. Is that still current?
-and-
Please add a screenshot of your DNS server page at https://IPFIRE:444/cgi-bin/dns.cgi or menu Network > Domain Name System.
EDIT: Update above command line