It works correct on my system.
I created a sample database with a button bar, slide control, and text box. First test : the text box was outside the slide control, button bar inside slide control, and I sent the text box to the back and the text back was then behind the slide control. Second test : place text box in slide control and I sent text box backwards and the text box was then behind the button bar.
Try this: Right Click on that button bar & select "Arrange", then select "Bring to Front".
Figured it out... the slide control was barely in the header making it always appear behind objects in the body.