I had the same problem. I registered a domain at Google Domains. Then I configured the domain at CloudFlare. At first it worked OK then I started getting SER

I had the same problem. I registered a domain at Google Domains. Then I configured the domain at CloudFlare. At first it worked OK then I started getting SERVFAIL. I found the problem was there was still DNSSEC configuration set up at Google Domains. I deleted that and everything worked OK. Cloudflare was not at fault in my case.