My only suggestion would be to measure the Vbat signal (between U14 and U15 on the schematic) and see if that is 1/2 the actual battery voltage. If not, then this voltage divider circuit is defective, or more likely, the reverse voltage damaged that input-pin on the STM.
If the STM can’t see the battery voltage, then it will pull down the CHRG output signal and this could possibly force an overcharge condition to the battery and also keep the red LED on.
Does the red LED turn to yel/org when the unit goes to sleep?