Okay. I don't exactly know which update changed this behavior, I didn't want to play with rollback, so I Photoshopped an illustration how it looked like. The difference is not hard to see, because this is the behavior of all other Windows applications that stick to the classic look.
See my attachments.
how_it_looks_now.png shows how a focused tab button looks like now.
how_it_looked.png shows how a focused tab button used to look like.
hover_how_it_looks_like_now.png shows what it looks like when I move the mouse over a tab button. Old behavior didn't change the border of the tab button when I moved the mouse cursor over it.