Validating PHC operation on Microsoft VM

I'm running chrony 4.5 (which is the current default) on a commercial
Microsoft Azure VM (Linux myVm 6.8.0-1021-azure).  By default a PTP
hardware clock is used as a time source:
chrony.conf has no servers defined

refclock PHC /dev/ptp_hyperv poll 3 dpoll -2 offset 0

No external servers are defined.  This is the default configuration.

Current time source:  hyperv_clocksource_tsc_page
Available time sources: hyperv_clocksource_tsc_page hyperv_clocksource_msr
acpi_pm
This is the default setting.

AFAICT chrony is happy, but is providing inaccurate time: Time is in error
by 30-50 msec with substantial jitter.

Dropping the PHC refclock and using nearby stratum 1's give much better
results.

Any suggestions for further debugging?