I have been using NTPD reference clock 127.127.20.0 with PPS for years without issue. I recently attempted to upgrade 21.02.3 to 22.03.0 and my clock was completely unstable.
After investigation, it appears my stable clock is using ntpd_4.2.8p15-3_aarch64_generic.ipk and the unstable clock using ntpd_4.2.8p15-4_aarch64_generic.ipk.
The "-4" package appears to have removed support for ATOM reference clock 127.127.22.0. and also does not work with ppsapi in NMEA 127.127.20.0.
The source code for ntpd has not changed, so these issues appear to be compile time changes.
I have manually installed -3 onto 22.03.0, but ppsapi for NMEA 127.127.20.0 fails upon starting NTPD. I have also re-installed 21.02.3 that loads -3, same failed result.
Thus may have been another version before -3 that was working even better, or -3 was recompiled with the same version string as I have two systems, one running -3:
Installed-Time: 1662663865
where reference driver 127.127.20.0 works with kernel ppsapi and the other installed from -3 with a date on downloads.openwrt.org Sun Sep 11 07:25:12 2022; the later cannot use ppsapi.
I have e-mailed the package maintainer but not gotten a reply.