Periodically, my domain started resolving to a Fortinet address: 208.91.112.55. I haven’t configured anything on Fortinet’s side, nor do I use any of their services. In Cloudflare, this record for the domain is of type CNAME and points to AWS CloudFront.
What feature, service or problem is this related to?
Since the domain in question was not shared in the post, it is hard for us to give specific advice. However, if the domain is resolving to a Fortinet IP is because somewhere there is a DNS record pointing your domain to that IP.
Different public resolvers might give different responses, depending on the server that they query for the response. You can use DNS checker to see the resolution for your domain from many different resolvers https://dnschecker.org/
You can also run a dig example.com +trace +nodnssec to see which nameserver is answering with that Fortinet IP.