An update from my 30kW 2018 model..
Yes, I have had Kia Assistance two times at home because of a flat 12v battery
First time I had no idea... It was almost dead... I was able to open the door, the cabin light started to flash and then it was dead.
When the Kia Assistance came later to pick up my car everything was fine... and he told me maybe the 12v battery was flat and recommended me to charge it.. Ok, gave 24Hour charge with external charger
Then a couple of days later it was dead again and this time totally dead, had to open the card with the key, connected the charger again and called Kia Assistance and asked them to pick up the card and ship it to the dealer, the dealer called me later the same day and told me they installed a new battery and updated the software.
Was told it had an update for how the car charge the 12 battery when charging the main battery..
ok.. In both situation the car had been connected to the charger at night to get it up to 100%
So the after a month.. drove over to a shopping center, shopping for 1 hour, came out and almost dead again
Low battery warning and 11.6 volt on the battery, ignition on but no start
Connected my jumpstart and it was ok again
Then to my next observation
When driving in daylight there is only 13.1Volt in the car but if I turn on the headlight it goes up to 14.1Volt
So after this... I always use headlight to give the battery max volt like generators in Petrol/Diesel engine does
Knock on wood.... No more issues so fare
So when it comes to the DC-DC converter I have a full log of a 4 hour 15 minutes charge on my 230Volt/16Amp charge station and you can see that the DC-DC is not charging the 12V battery with +14Volt all the time and at the end it seems to be something taking the battery down to 12.4 volt when the charge end.