The power supply circuit of the XIAO nRF52840 is probably as shown in the link below, and looking at the PPK2 current waveform, I suspect that the battery charge controller is consuming a large amount of current to detect whether the battery is connected.
(The load regulation of the voltage regulator is 3-15 mV)
Can you use a 3.7V rechargeable button-type lithium battery?
‘Charging characteristics of XIAO_nRF52840 and XIAO_ESP32C3 - #2 by msfujino’