Comment by the_biot

debug times out as well. curl output:

   flake: curl -v https://fly.io/blog/ -sS -o /dev/null -D-
   *   Trying 2a09:8280:1::a:791...
   * TCP_NODELAY set
   * Connected to fly.io (2a09:8280:1::a:791) port 443 (#0)
   * ALPN, offering h2
   * ALPN, offering http/1.1
   * successfully set certificate verify locations:
   *   CAfile: /etc/ssl/certs/ca-certificates.crt
     CApath: /etc/ssl/certs
   } [5 bytes data]
   * TLSv1.3 (OUT), TLS handshake, Client hello (1):
   } [512 bytes data]
   * OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to fly.io:443 
   * stopped the pause stream!
   * Closing connection 0
   curl: (35) OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to fly.io:443

Seems to work when forced to IPv4 though.