Another bug report with similar symptoms is https://bugs.launchpad.net/ubuntu/+source/bind9/+bug/1954854 - although that was from Dec 2021 and occurred soon after an upgrade from Ubuntu 18.04 to 20.04, whereas most of my servers have been running Ubuntu 20.04 for 6 months to a year.
Another bug report with similar symptoms is https:/ /bugs.launchpad .net/ubuntu/ +source/ bind9/+ bug/1954854 - although that was from Dec 2021 and occurred soon after an upgrade from Ubuntu 18.04 to 20.04, whereas most of my servers have been running Ubuntu 20.04 for 6 months to a year.