This is a known issue (well, at least, known by me). Although I was never able to pinpoint the root cause, I strongly believe it is because of some nasty upstreams Cloudflare employs in their PoPs. AS6663, for instance, inspects and filters outbounds connections from Cloudflare’s servers to origins that are blacklisted by Turkish government. They are dualhomed to Cogent in Istanbul, but still keep routing over this offending upstream (172.69.182.0/24 - bgp.tools).