- As you mentioned, "On Preview" so what's behaviour in real time?
- Is it happening in all the tab group?
- Login with OOTB credentials "firstname.lastname@example.org/install". Navigate to, DesignerStudio->User Interface->Gallery->UI Elements->Layout group. Observe whether it's working correctly here? If yes, click on "View design-time configuration" link on the top right corner to see the correct configuration of tab groups. You can replicate in your section to implement your business use-case.
Hope it helps, kindly notify by marking this post as answered/helpful if it does for larger Pega audience.