Very true... I used to be with Sky here in the UK, and at the time they were running a transparent proxy on port 53. Changing DNS providers made no difference to the dnsleaktest results. Don't know if they still do that now.

I'm now with a different ISP, and anyway have PiHole handling DNS queries on most devices in our house. It forwards DNS requests to dnscrypt-proxy running on the same Pi, which uses Quad9 over DoH.