Background application groups de-group for a split second on click

Grouping behavior is set to be like Windows default.
So basically when clicking on an grouped button of an unfocused application, it ungroups the button (or rather shows the animation for it I'd say) for a split second, goes back to normal and then proceeds to show the thumbnails.
Only a minor annoyance, really, but still throws me off a bit.

I can't really say when this started happening, but I was running a Windows 10 install that didn't get any updates before (I couldn't really fix that so I went with a reinstall by now), so I guess it worked back in RTM, if that's of any help.