Widget designer repainting issue with button box (Gtk-Style, Form editor within Qt Creator only)

Steps to reproduce:

• create a form widget class
• drag a button box into the widget
• click on Cancel of the button box -> Cancel is highlighted
• click on OK of the button box -> Cancel is still highlighted
• move the mouse cursor out of the button box -> Cancel and OK are highlighted
• move the mouse cursor back onto Cancel -> Cancel gets unhighlighted
• can be done vice versa

Expected behaviour:

• the button that has been clicked last inside the button box is highlighted and the other one isn't