Control: tags -1 + wontfix confirmed > pdns-server in Stretch fails to answer TCP queries made by clients via > backend recursors reached over IPv6. I discovered this setting up > cloudflared, using it as the recursor, but have also replicated on a clean > install with Google and Cloudflare's public DNS servers.
Indeed. The recursor proxying code for TCP is not IPv6 enabled. Good that this feature is already gone in 4.2. Cheers, Chris