Very small currents cannot be measured by the current sensor, so the BMS will not update the SoC. Only when one of the cells is below Vmin, the SoC will be recalibrated to 0% because this is a known point to be empty.
Make sure the pack is charged regularly so the SoC is recalibrated to 100% when full.
If you are not going to use the pack 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.