0.pfsense.pool.ntp.org doesn't work on IPv6 only installations
I debated whether this should be considered a bug or a feature. I ultimately decided it should be considered a bug seeing as how NTP won't sync if you only have IPv6 connectivity.
The only zone that contains IPv6 pool members is 2.pfsense.pool.ntp.org. CentOS 8 has done the same thing and are now using 2.centos.pool.ntp.org by default. pfSense should be configured to use 2.pfsense.pool.ntp.org by default so IPv6 only hosts are able to obtain the correct time via NTP.
See the thread on the NTP pool forum for an explanation as to why only that zone is IPv6 enabled.
Switch default NTP pool server. Fixes #9931
2.<x> pools contain both IPv4 and IPv6 hosts.