Usually this indicates that you have a header layout part and that the tab control touches or crosses the boundary line between header and footer. Move th tab control down until this is no longer the case and this issue should disappear.
Hint: Since tab controls are most often used on form view layouts, you may find that you can remove the header layout part from this layout and then this issue is no longer possible on that layout.
I double checked and the only part I have on this layout is the body and the only view I have available for the layout is the form view.
Make sure that your tab control is not locked and that the boundaries of the object you are placing into the tab panel are fully inside the limits of the tab panel. One quick check to see if the object so placed has become part of the tab panel is to move the tab control a few pixels to see if the object you placed on the tab panel moves with it or not.