Hi Sarkis Matossian,
This is quite unexpected!
Can you confirm that there is no overlay (a transparent visual object) over the tabs?
Are the IsEnabled / IsHitTestVisible properties both true for the control and the items?
Are there other visual items in the headers of the tabs, apart from a TextBlock?
Are you using a custom ControlTemplate? If you are, make sure that there is an element in the template called "wrapper" and that it is the root Grid of ControlTemplate or something similar.
I created a TabControl with the 8.12 version of the controls and I could not reproduce the issue. Anything else that could help us identify what may be going wrong in your case is welcome.
the Telerik team