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I haven't studied all the details of the M5Dial schematic and I don't have one for testing myself so these are just some suggestions shot from the hip:
Check what the power consumption is when it is powered down. There should be a similar function called powerOff(). Referring to my previous explanations in your other thread about the RTC, the power consumption after powerOff() and timerSleep() should the same. If you see a difference there might be a problem with the timerSleep() in the firmware.
You could also try to cycle the 12V externally and see what the consumption is before the M5Dial is switched on. This however, will only work if the power logic is not made so that the device switch on as soon as external power is applied.
Above tests should be done without an "internal" Li-Ion battery attached, but you could also try to measure the consumption from this battery without the external 12V power source. This will tell you whether the power consumption is from the power circuit for external power (buck conv. and Li-Ion Chg.) or from the main part of the M5Dial (ESP, display etc). Again you should measure the same three things as above: powerOff(), timerSleep() and just after having connected the battery.