I have myself done more testing and I notice that when dmesg says:
rtc-pcf2127-i2c 1-0051: pcf2127_rtc_read_time: read error
[ 1.609983] rtc-pcf2127-i2c 1-0051: hctosys: unable to read the hardware clock
the RTC time is not transferred to system clock at system boot.

But I tried:
sudo hwclock ...

I am testing a RevPi Connect+ unit in my office WITHOUT internet connection.

Kunbus claim the RTC will hold power for 24 hours, however my time is lost when the unit has been down for much shorter time than that.

When I start the unit after a short time without power (just tested now with 30 ...