Error 521 for IPv6 only origin server

My origin server is reachable only via an IPv6 (my Internet Provider has only shared IPv4 addresses, making my resources unreachable over an IPv4 address).
I have configured only an AAAA record in my DNS zone (with no A type record).
Theoretically CloudFlare should be able to reach my origin server by name (from both ipv4 only and dual stack ipv4/ipv6 networks), as I can when using the IPv6 address from different networks over the Internet.
But via CloudFlare there is always a “Web server is down: 521” error.
There is no firewall issue.
A network sniff shows there is no activity between CloudFlare and my origin server when reaching by name.

Error 521
Ray ID: 5edcef4b7be5acfa
2020-11-06 06:59:57 UTC

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