You are supposed to add them to EACH platform THAT THE base game was released on, not to a platform the base game was NOT released on
DLC must be added to EACH PLATFORM SEPARATELY, not to a PLATFORM that the BASE GAME doesn't exist on
In other words, you must add them ONE BY ONE to ALL the platforms the game came out on. tHEY WILL NOT SCRAPE otherwise.
For example if DLC exists for both PC and XBox, you have to add them to BOTH OF THOSE platforms SEPARATELY/INDIVIDUALLY, not to a completely different one.