Update: I am investigating the DLC issues but cannot reproduce it in my build. Can you check if you have the file called "8csgDLC1.ini" in the config/dlc folder?
As a temporary fix, if we can't find the cause:
1. Navigate to the "media/stages" folder inside the game folder
2. open "stages.ini" and add the line "../dlc/stages/8csg 0" after "training 2". Make a backup copy of "stages.ini" before doing this to avoid any issues.
This should add the stage to the game too, it's a hacky way but guaranteed to work.