the main openntpd server on my network had accidentally been without a connection to the internet for several weeks and thus unable to connect to its configured remote clock sources. Oddly enough, "ntpctl -sa" would still output a status of "clock synced, stratum 2". Other machines on the network using this server as their clock source would report a stratum of 3.
I would have expected openntpd to consider its clock as out of sync after some timeout and notify downstream hosts accordingly. I'm not sure how this case is intended to be handled as per the NTP protocol. I have, however, seen the ntp.org ntpd report a stratum of 16 in this case so clients further downstream can detect that there is a problem.
I have since reconnected the affected machine and things are back to normal. Let me know if you'd like me to perform any additional tests. It appears that this can be easily replicated by disconnecting a single openntpd server and watching the output of "ntpctl -sa" on that machine.
Thank you, I can reproduce this. It's a generic problem with OpenNTPD and not specific to the FreeBSD port. I will raise the issue with upstream.