shift-tab backward focus change stucks

In attached ui when alt+tab is used it works good and moves through all widgets. But when stopped some where and uses shift+Tab then this does not goe back through entire controls and stucks in between.

Run Ui fle Ctr+R in Designer
hit Alt+tab to get thefocus in groupbox3
Hit shift+Tab focus will stuck in groupbox3 controls and the buttons at bottom.