A few observations.
The Design Studio does not display in the HUD when it has been researched, unlike Marketing which does. I would like to suggest that this does appear in the HUD GUI.
I have also noticed that when a body style research is completed, that you are not prompted to research another body type. Unlike the main Research feature which does.
A final thought. After researching SUV you can research Pickup Truck, and both these vehicles show as two distinct models. However after researching Sports Body and then Open Top, you are still left with only Sports Body? These should be two distinct models currently in game.
Again, thanks for a great game Cliffski