It happen sometime when one or few tab objects are displaced. Please check the objects lies inside that tab. I think some of your tab objects have crossed the tab area due to which it looks distorted.
Also make sure that no part of your tab control crosses or touches the boundary line separating the header from the body. Since tab controls are most frequently used in Form View layouts, you may want to move all objects out of the header and delete that layout part from your layout so that this issue cannot occur.