Hello @patf
I just ran a test with an empty program (empty setup and loop functions) and yes, with that M5StickC can be turned off, but not turned on again via power button.
Including M5StickC.h and instantiate M5.begin() fixes the power button and the device can be turned on again with a short press.
Background info: M5.begin() calls M5.Apx.begin() which then properly sets up the PMU AXP192 chip in the M5StackC.
Cheers
Felix