Well, you didn't reported this specific bug with negative temperature before, I didn't know. You tried to explain me that AC was heating, and that you could heat the bus with no fan speed which was nonsense. But I suspect this has somethng to do with you pressing the AC button. This is not normal behaviour indeed, but there is probably som sign issue in the script. But just stop pressing the AC to heat the bus. AC does NOT mean heating...
The freezing was a new thing I saw, because it was the first time I enterned the bus at minus temperatures. Before that it was at least above 0. I would assume that it has something to do with engine temperature, which is also first negative when spawning the bus. And as engine of course heats up the bus we see such a side effect when we are below 0. The AC button was not touched by me, and the freezing happened in busses with AC as well as without. In order to get the bus warm I had first to turn down the vent and make sure it is not running (not even al little bit), then wait until the bus gets above 0 just from engine heat, and then I could turn on the vent without worrying about freezing.