I think I can confirm that there's an issue with cf's https://1.1.1.1/help website, I've setup a stubby on a Windows 10 VM to test that site with DNSSEC on and off, so a setup that is completely different in terms of operating system, except for stubby.
In the default setup of stubby I only changed the upstream servers to be only cloudflare, turned off round robin, and tried with DNSSEC on and off and the results are the same, when DNSSEC is on the site fails to recognize the connection to 1.1.1.1 and DoT, when it's off it consistently recognizes, even thou in both cases it displays AS name as Cloudflare.