So its a 15 cell battery as compared to the typical 16 cell, 48V battery. I prefer to think about batteries on a per cell basis then multiply by the number of cells which would normally be 4, 8, 15 or 16.
The battery is charging at 51.02V which is an average of 51V/15 = 3.4V per cell. This correlates nicely with the individual cell readings very close to to 3400mV. This is right on the threshold, but not quite high enough to fully charge the battery. I would recommend increasing Absorption (Constant Voltage) charge setting to 3.5V per cell (52.5V) let the battery charge until the current starts to fall off again. At that point recheck the individual cell voltages and if #8 is still a little low let the battery charge for a few hours and recheck #8. The balancer should correct this.
Regarding SoC, I would suspect the the battery is actually at a higher state of charge than 66.4%, assuming the voltage reading of 51.02V is accurate. The BMS is not intended to be a long term, precision current monitor and coulomb counter, it actually relies on being recalibrated to 100% at the top of every recharge cycle, usually at some set Voltage value. Not sure if you have access to these values or can change them. In any case, the charging voltage has to reach the reset value for the SoC to jump back to 100%. You can safely increase the charge voltage up to 3.6V per cell. (54.0V) to force the BMS to reset SoC to 100%. This is the typical behavior of a BMS, I've not personally used a Rosen Brand battery.
