Firstly, when you hover cursor over a tab, it isn’t much different from a neighboring tab.
The difference is quite visible though, that’s enough I believe.
Secondly, when you hover cursor over a tab, during animation for a moment, it becomes the same color as a neighboring tab.
I don’t see it as an issue, being a transition, I think it’s ok. Moreover it lasts a fraction of a second.
But why StackSwitcher should look like a regular button? In the previous style, I didn’t see anything wrong.
The function of stackswitcher and buttonbox (in this particular case) is the same, so I expect that also the look and feel is the same.