Content at this address is not displaying, and when performing dig, the result for this domain is NXDOMAIN. Dig for fbcdn.net gives the actual ip address and thus works.
When using another DNS (even one of the ones at public-pi-hole.com which also run Unbound), the problem does not exist.
Only issue I immediately noticed was the ipv6 address in setupVars.conf, so updated that. ( updated debug token with only that changed is here https://tricorder.pi-hole.net/0io088ev77
). No effect on dig on this specific domain though afterwards.
However, editing /etc/unbound/unbound.conf and taking out these parameters solved the issue:
qname-minimisation-strict: <yes or no>
QNAME minimisation in strict mode. Do not fall-back to sending
full QNAME to potentially broken nameservers. A lot of domains
will not be resolvable when this option in enabled. Only use if
you know what you are doing. This option only has effect when
qname-minimisation is enabled. Default is off.