I have installed tvOS 12 since beta 1 and I have never got the NASA screensavers. I checked all of them swiping while in preview, if there is a new screensaver is the snowy one (not sure if it was there in 11.4). Nothing from space. Xcode reports about 5GB free, and then being the flash memory a temporary buffer it shouldn't be an issue. Is there a way to force their download? Obviously I have set the screensaver to download it daily. I really don't feel like I want to do a full reset (although it isn't so bad like in Android TV, basically at most everything goes back to what it was before. Few logins apart, like in Netflix, maybe).