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That’s very strange.
Are the installed batteries LiFePo4 type?
Please, firstly measure a couple of cells, one on the main board and the other on the daughter board, with the battery system powered off (even the TWRPS-LBS-P) and without any load.
You should measure 3.3-3.4V.
Then, with the battery system powered on, please measure the voltage on the MCU. You can measure the voltage across C2 at the bottom layer of the main board. You should measure around 5V.
You can also check the value of the resistors at the input of the ADC of the MCU on the bottom layer of the main board. R16/R23 are 10Meg and R13/R22 are 1K.
Please, check their soldering.
The battery overvoltage is controlled by the MCU, which seems to fail in your case.