I have 2 theories:
1. According to recent information the Mini uses CATL 811 cells. CATL had a lot of problems with that chemistry (web information and news) and maybe thats the clue. 5 months ago i requested information from BMW over the email with direct question what batteries are being used in our cars. The only answer i received is that it uses Battery type S14, generation 4.5 - no answer regarding the chemistry.
2. Most EV use aluminium case for the batteries, but the Mini uses regular iron. Maybe corrosion is starting to build up on some vehicles battery pack and moisture is getting inside....
Anyway, i think this software update is only tool to keep an eye of our cars BUT not a permanent fix. So if you see that your dashboard lights as a christmas tree or car is preventing from charging over 30%, then you have a problem. My car is only 9 months old and my first EV.
Best Regards!