It seems I missed the original forum thread that detailed this same NTP cache issue. It offered a work-around by re-enabling checkmk using xinetd in place of the systemd check_mk.socket service.
(NTP Time service check problems after 1.6 to 2.0.0p12.cee upgrade - #9 by JonR)
We tried this and it worked for us with all the affected systems with stale ntp caches (chrony) now updating correctly and no further errors reported by checkmk.
Just for general comparison, as mentioned previously, not all of our systems displayed the error.
They run a mix of Oracle Linux and Red Hat Linux 7 and 8 with all systems running chrony for time synchronisation.
We currently run checkmk rel 2.0.p22 on all systems.