The gas mix shifting to 50% is a safety thing. IF you dive a mix other than 21% the computer requires you reset the mix on each repeative dive. Basically it's a way to penalize you significantly (and keep you safer) if you don't tell the computer exactly what you are diving.
That said, I get dinged on this exact thing all the time.
Yep, that's what I've heard. However, I don't dive anything other than the gas mix the Creator gave us. Have never dived Nitrox, but somehow I must have reset the mix thanks to heavy fingers on the buttons!