Using DHCP DNS server
Although I set up the AmpliFi mesh router to use a couple of specific filtering DNS servers, it still shows a third one from the ISP, defeating the DNS filtering at home.
It seems that, having specified my own IPv4 DNS servers together with enabling IPv6, results in the IPv4 servers being used, as expected, and an IPv6 DNS server being added to the list through DHCPv6.
Since IPv6 is preferred over IPv4 by many a modern OS in the client systems, it results in my IPv4 DNS servers not serving most address requests. Perhaps being able to manually specify my own IPv6 DNS servers would help mitigating this issue, alas only IPv4 addresses are currently allowed to manually specified.