It has been known for a while that ISP DNS is incorrectly shown in the router UI when looking at WAN details. With the introduction of DoH in fw 8.2 the DNS info shown for WAN details is now doubly wrong as it does not include the DoH setting which over-rides any specified IPv4 addresses for old insecure DNS.
The screen shot is from a Balance 20x running fw 8.2. The red arrow shows the ISP DNS. The green arrow shows Quad9. The router is using Cloudflare with DoH.
Here you see the DoH setting for Cloudflare.