I think your battery is the likely cause. It probably has one or more sick cells. You can have a look with the SoulEVSpy Android app. You will need a bluetooth dongle to do that - KONNWEI KW902 is recommended.
Alternatively, take it to a KIA dealer, and ask for a SOH (not a SOC) - if it is less than 70% you should get a replacement if still under warranty.