I have a similar issue - but slightly different behaviour:
I use the built-in RTC and it works fine with WIO connected to USB-C power.
I wrote a small routine that displays the RTC time on the WIO screen. It works fine as long as power is present. The RTC time is set at compile time, using DATE and TIME .
Problem: When I disconnect power for (example) 15 minutes, and reconnect to power, the clock starts running again, but is 15 minutes behind.
So the WIO’s RTC module seems to have some kind of memory for the current time, but seems to stop running when power is absent. Schematic doesn’t tell me much…I see no RTC… the name of the library is RTC_SAMD51.h, so I assume the RTC is somewhere inside the processor.
I know that I could re-sync the RTC using NTP over Wifi, but that is not a good option as I plan to use it outdoors with no Wifi. Any ideas on how to keep the RTC running?