Cause:
This is expected behavior 
This has to do with the way BGI works. You will also notice that the control panel preview windows behave the same way. The reason you are seeing the "expected" result in the large preview window is that is simulating the projection window and shows the full invocation of the BGI. 
This has to do with the way the BG Improv is invoked - unlike a "regular" motion/image background there is an additional step required to actually display the selected background using BGI - even if there is a default image/video selected for BGI, the default will not being shown in the preview strip. This is done on purpose to prevent bad things from happening during the editing of BGI properties as the BGI globally affects anything that is displayed.