I had this happen to me once. It’s very frustrating, though I am a CF customer who uses CF to protect dozens of sites I admin.
In my case, what happened is that my home ISP provides me service behind a NAT firewall, so I am sharing the public IP address I use with a lot of other customers of my ISP. One of those other customers was running a spam operation and the IP address got listed on a spam blacklist. As soon as I started seeing CF captchas everywhere, I knew something was up and I checked my public IP on a blacklist checker and sure enough it was on a couple blackhole lists. I brought this to the attention of my ISP and they immediately switched the NAT provisioned for my service to a different IP address, found the spammer and kicked them off the service.
There was really nothing that CF could have done about my situation as it was totally outside their control and they were simply providing the service that I and many other website operators have hired them for - protecting the site from bad actors. In my case, I got put into that category because of someone else’s actions and I had to do a little work and get a little help from my ISP to fix it.