I have MainLevel in my Release build, and a map DLCLevel in a Content-Only plugin called DLCPlugin1.
I build both using the Project Launcher and use these settings:
Release Profile - Imgur: The magic of the Internet
DLC Profile - Imgur: The magic of the Internet
I then copy over the DLC pak file to the Release build’s Content/Paks.
Now when I run the game, it shows MainLevel, but when I use open DLCLevel in the console or try calling OpenLevel via blueprints. It just silently fails.