As DNSSEC adoption increases, the number of customers who move domains without first disabling DNSSEC at their existing registrar increases as well. Most recent example here:
Cloudflare should check for existing DNSSEC records and display a warning in the setup process (and overview for the zone) for this.
New most recent example = this guy 
Added a vote for the feature request. There should absolutely be a warning given the implications of the problem.
With Google Domains shutting down, I migrated multiple domains to Cloudflare. Two had DNSSEC enabled and are now broken. I imagine others may migrate from Google Domains and encounter the same issue.
I opened a support ticket and was directed to the forums, which I reviewed before creating the ticket. The general consensus for a fix is to correct the problem at your registrar, and Cloudflare is now my registrar.
To be fair, I caused the problem because I didn’t turn off DNSSEC before migrating. However, implementing a warning message seems like it should be a given and feels like it would be a light lift.
I’ve used Cloudflare for a long time, and it’s an amazing service. So, I’m not super bothered, but if I were a new customer in the same scenario, I’d feel pretty bad about my initial experience with my new registrar.
You should be able to get it fixed with a registrar ticket. It would be great if such occurrences could be reduced with a simple check and response, though, wouldn’t it?