I have time.nist.gov as an NTP Time Server. time.nist.gov resolves to "2610:20:6f15:15::27". On the NTP Status page, the IP address shown is "2610:20:6f15:15" (missing the "::27"). This timeserver also shows as unreachable on the NTP Status page.
Either this is a display bug, and the NTP server being unreachable is a coincidence or pfSense is passing an incorrect IP to the NTP service.
Are you on a current version? That should be fixed if you are on 2.3.3 or later. https://redmine.pfsense.org/issues/4815
I am on the current version, yes.