Makes no sense to me. But as I said before you can get multiple design bodies with this workaround:
Select "copy as construction body", hit OK, select all the solid bodies under "construction bodies", right-click and select "toggle design/construction".
GTAC may be my next option. If just to ask "why?".
GTAC says this command works as designed. So if you want to IPC an assembly with a multi-body part as a result use construction copy and toggle to design.