The BMS is a software algorithm used in a computer that measures the voltages and temperatures of the battery pack.Reason I said 3%, is that I'm assuming the battery pack of the Soul 30kWh is the same as the Ioniq28kWh, which is close to 31kWh.
Having the same BMS is completely independent of the battery pack.
The Kia Soul 30kWh and the Hyundai Ioniq 28kWh have completely different battery packs.
Made by different companies and with very different chemistries. Buffer sizes and total capacity are as yet unknown for both.
pmiddeld - try using the soul spy app to show the battery cell voltage heat map. I suspect one of your cells may be going bad.