Why is lvl 50 dweller at the bottom of the list?

I have the same issue. It seems to be related to unique characters.

Your Colonel Autumn is one of them right? You get him from one of the lunch boxes, right?

It seems to be a bug. I have three people from lunch boxes. One with lvl 40, one lvl 50 and one lvl 1. The lvl 1 leveled up and is in front of all lvl 1 and behind all lvl 3 and above. As it should be. Lvl 40 was between the lvl 1 people, just like in your case. After leveling up, he gets back to the lvl 40 people.

Due to the fact, that there is a level cap at lvl 50, your lvl 50 character won't be able to level up. This means he will stay there until a bugfix may be released.

Not the answer you desire, but it's your answer. Sorry.