I was just messing around with the units and the "Auto" setting is a little strange sometimes. It seems like some of the time it chooses the unit with the smallest integer. And other times it is arbitrary, like selecting lunar distance for semi-major axis when, for example, Earth's would maybe be best displayed as 1 AU.
All this to say I'm not really sure how the Auto setting works, I'll ask the team.
Being able to set the default unit type for each property is also a good idea, though. Is that what you mean? Or do you mean not having to reset the unit type each time you close and open a property window? That alone might do a lot to help solve the issue.