Something I changed just made it work. I deleted the sky dome (and fog) from the sub level. And now it seems to be working. So maybe the sub-level can’t have a sky dome with clouds. And the double fog was overrunning both levels. But otherwise it seems to be working, by way of bypassing the Levels Tab and Add Existing sub-level. And instead just drag and drop from the Content Browser into the parent Persistent Level, and maybe also setting Always Loaded. I can now see from the outside into the sub-level, and also from within the sub-level out into the main level.