Odd for sure.
I’m fine with the current fix, but I will likely continue to check up on it (i.e. remove the CSS fixes) as I update plugins/themes in the future – perhaps this issue affects more than just GeneratePress. In fact, I’m certain that it does.
I noticed one icon in my admin bar that also goes off the vertical center, but only when I’m viewing a web page (not when I’m in the admin back end).
If I ever find out what the culprit is, I will be sure to update this thread.