In Unreal 4.8 Preview 3, when creating a new level in the Levels window, it is not possible to give those levels a name. The naming screen is skipped and the level is called “Untitled_*” and automatically placed in the “Temp Content” folder on saving.
This problem did not arise when using Unreal 4.7.6. I have attached diagrams to illustrate the problem. The test projects were blank Blueprint-only projects.
(Incidentally, it would also be very helpful if it were possible to rename levels from the Levels window once they have been created.)
Unreal 4.8 Preview 3:
Unreal 4.7.6: