Since Yahoo changed things around on the new version some time ago, I have had issues with it's operation.
In most email programs I have used it works like this. You are on a email and want to sort by sender or maybe subject line, so You select that and you see the email you have highlighted at the top of the screen and whatever you have searched for above or below that.
So if I am looking at am email from Joe Sixpack and want to sort all of his messages to find another one he sent, I hit the Sort By Tab and select sort by sender. Yahoo shows all of the messages that start with J, but the one I am looking at is not at the top. So you have to scroll thru to find it. It seems lame in how it operates compared to other email systems.
I hope I explained what I am seeing