BQ77915: BQ77915's CHG pin always output minus 2V.

Hello guys,
I have designed a 6S BMS circuit using two BQ7791506 ICs in stacked configuration.
Their DSG pins work fine and output about 10V for both upper and lower ICs while their CHG pins output -2V for upper IC and 0V for lower IC.

I have checked everything in table 9-5 and found nothing unusual. Each cell voltage is 3.7V so OV and UV faults are not the culprit. For upper IC, its SRN and SRP pins are shorted to VSS, so OCC1, OCC2, SCC faults are not the culprit. I have checked the TS pin and found out its voltage is 1.56V which is 52% of VTB (3V), which makes all OT faults not the reason for both CHG pins output minus voltage.

Please help review my schematic.