Warn when trying to change DNS records for a delegated subdomain, which won’t work.
I had a subdomain delegated to the Hubspot Knowledge Base, I do not know how this happened (since I didn’t add any NS delegation to Cloudflare for that subdomain), when I wanted to change the DNS, if proxying via Cloudflare was still on, the request would have reached the delegated 3rd party (Hubspot) instead of my new IP that I added in the CF DNS records.
There should have been a way to warn me that the subdomain is delegated and contact the 3rd party. Although I do not understand how the 3rd party had access to where my trafic reached without having any Nameservers configured to reach them and the only thing I added was an A record to their IP that after changing would still point to them because I had it set up in their platform as well. Very peculiar!
I’m not sure what this has to do with NS records or subdomain delegation.
It sounds like this is a Cloudflare for SaaS issue where you used their platform and, as they use Cloudflare themselves, they took control of the configuration for that hostname. Due to hostname priority this takes priority over your own configuration so the platform works as expected. Unfortunately this has the side effect of your site not going where your DNS records actually point if it is proxied.
The provider is supposed to remove the custom hostname when you are no longer a customer but many don’t. Cloudflare are aware of this issue and have said they are working on making it more obvious to users when this is causing issues.