Very small currents cannot be measured by the current sensor, so the BMS may not update the SoC. However, the BMS will always sync the SoC to 0% when a cell has reached Vlow/Vmin.
Ensure the battery is charged regularly so the SoC is recalibrated to 100% when full.
Note: from firmware v3.3.0, the BMS is able to correct the SoC when the battery is at rest. The BMS will automatically verify the SoC and update, based on the cell voltages.
Tip: If you are not going to use the battery for a longer time (for example because it is winter), you can disconnect all loads so the pack will not be drawn until empty. You can also connect a small trickle charger to prevent the batteries from being discharged during standby.