DNS will return DNS records, but never any web page.
Are you saying that Cloudflare's DNS servers are answering DNS requests with your router's IP?
I guess you are using mydomainX.com as a placeholder for some other domain names, but would those be publically registered domains?
Can you show some log output from /var/log/pihole.log for both some lookups answered locally as well as some of those unexpectedly forwarded to Cloudflare?