Why is ipv6 dns entry inserted by Mac?

I recently found that my Mac (Monterey) was using an Australian censored DNS instead of the ones I had specified in my routers DHCP settings. I noticed that there was an IPv6 DNS entry in the Mac's network settings in addition the ones I specify to be allocated from the router. The router has IPv6 disabled. I think the DNS was Telstra yet I'm with a Vocus ISP.

Changing the Mac IPv6 setting from Automatic to "Link-local only" got rid of the rogue DNS entry.

I've only just noticed this behaviour so it's probably been introduced with Monterey or a recent update of it. Has anyone else noticed this? Why has Apple started doing this?