After returning from vacation and doing some more experimenting (this time not in a rush), I think I have identified and resolved the problem.
In short: "Pilot Error".
I inadvertently had the charge schedule override button pressed.
Before I left on vacation, I was observing charging starting right away, instead of being delayed. I then used smart phone UVO app to reset charging schedule, and hit the refresh button. After the interminable delay that UVO requires, the display refreshed and immediate charging observed. I (incorrectly) associated the immediate charge with pressing the refresh button, and though maybe something wrong with the app. Or my car. I spent several hours fooling around with this basic scenario, and then gave up and went to bed. It was the next morning when I went to start the car that I discovered the dead 12V battery that I previously reported.
Now, I don't know why the 12V battery discharged, but have to think it was some cause or side effect of the playing around I was doing.
Since I discovered that the override button had been pressed, and then turned the override off, I have had no problem.
I think it is a little unfortunate that:
1) The override button is right next to the charge port open button, which is used a lot. (Usually I forget to press it beforfe exiting the car, and then have to open the door, reach in, and press the button. It is very easy to hit the wrong button. So...
2)Too bad that there is no LED in that button, or some other indication that it is pressed. It really can "mess with your mind" if it is engaged, and you don't know it.
I am pleased that there is no problem, after all.
I would suggest to KIA that they either move the button (perhaps beside the climate override button) or put a LED in it or make it have a longer "throw" so that it is visually obvious if it is engaged.