To identify a parasitic cell I opened up the battery, disconnected all the balance leads and measured each cell group:
Each column is just a different day and arbitrary charge level that I measured. From these measurements my guess is either the BMS tolerances aren’t the best, or it’s a parasitic cell that’s pulling down the rest of the voltage in cell group 18. I think it points to a parasitic cell since the low group is further off at the low end of the charge. The BMS balances the cells only during charging according to Paul. I’m not sure if it only tries to balance during a certain voltage range.