I’m not sure whether changing between the sedan and compact is intended before unlocking Body Design, so I’m not sure which way this is supposed to be fixed, but when I open up Chassis Assemble (before unlocking Body Design), I don’t see the ability to change body design until certain (but not all) current tasks complete and another one starts.
Screenshots attached. Between these two screenshots, the dialogue box hasn’t been closed.
Further testing seems to be that the position of that button, and whether it’s there at all, changes depending on which part of the cycle you’re in.
