One of the great features of LFP batteries is also one of the big pains.
The voltage curve is nearly dead flat from 20% to 80% charged. You could have a cell at 70% and another cell at only 50% and they could measure within a couple millivolts. Anywhere in the middle of the charge/discharge curve, all the cells will be very close in voltage. But that does not mean they are balanced.
The only way to be sure they are truly balanced is to bring them up to full charge. As LFP cells reach full charge, they hit a "knee" where the voltage will star to climb quickly. That is the true full charge point. Ideally, you want all of the cells to get into that knee, and then if they are at the same voltage, the cells are then balanced to 100% charged. Anything over about 3.45 volts, with all the cells at the same voltage is great. Most LFP BMS units don't have a lot of balance current. As the first cell exceeds 3.45 volts, the BMS can apply a small load on that cell to slow the charging a bit to allow the other cells to catch up. But if the charge current is more than 10 times the balancer current, it may not slow the near full cell enough to keep it from running. This is what causes a cell over voltage error, even though the total battery voltage is still fine. IF you can't get to the individual cell leads, the only real fix is to charge at a vey low current where the BMS balance current is enough to nearly stop the high cell from charging at all. This can mean charging at less than 1 amp until they finally balance out.
