Cloudflare is not friendly to long domain names
GitHub
The web is not friendly to long domain names · Issue #2 · ccbikai/l...
I registered a long domain name loooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.ong and created a subdomain loooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.ooooooooo...
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well I know Let's Encrypt added something a bit back that was supposed to automatically fix that by changing it from the common name to just one of the SAN fields
but yea, not surprising
sounds like you were able to work around it for CF. I believe in the past that's what they've done manually (just adjusted CA)