Navigation does not work in sub-levels

Navigation does not work in sub-levels. If the Nav Mesh Bounds Volume in a sub level is not touching a Nav Mesh Bounds Volume in the Persistent Level, it will not appear or function during Play.

It makes no difference if the sub-level is set to Always Loaded.

Other unusual but probably related behavior is shown in the reporting user's video:
[Link Removed]

Reproduced in 4.7 Preview 4 binary and Main Promotable-CL-2418131

Additional AnswerHub threads regarding this:
https://answers.unrealengine.com/questions/171707/navigation-system-does-not-work-in-sub-levels.html

1. Create a new Blank project with no starter content
2. Save the level
3. Open the Levels window
4. Levels > Create New... > create a new level
5. Make the Persistent Level current
6. Add a Nav Mesh Bounds Volume to the floor in the level
7. Make the new sub-level current
8. Add a Nav Mesh Bounds Volume to the floor that does NOT touch the other Nav Mesh Bounds Volume
9. Press 'P' to show navigation
10. Simulate

Results:
The Nav Mesh in the sub-level is not there.