Danke @felmue ,
Your answer pointed me in the right direction.
The voltage between GND and BAT pins was indeed 4.0V+, so I went back to check my code.
I noticed that the official documentation for the AXP on the M5StickC only mentions
M5.Axp.GetVbatData() (which is what I used and what gave me weird readings) but the AXP code example uses
M5.Axp.GetBatVoltage() and the latter actually gives me the correct number.
What's the difference between these two? Is it just the