When importing some old worlds (.mcworld) it isn't imported correctly. The world folder is imported, but it doesn't contain all the files it should, and some are improperly renamed. E.g. level.dat is renamed to level.dat_old, thus making it non-functional.

I suspect it has to do with the new world format.

You can try this yourself by using the following steps:

1. Download this world: http://www.mediafire.com/file/6burczzt6c2ju5j/deepoceancity.mcworld
2. Import world to Minecraft
3. It won't be visible in your worlds list. Open following path and you will notice the world is imported, but contains incorrect files (e.g. level.dat_old) C:\Users\johnz\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftWorlds

It is possible to fix this manually by renaming level.dat_old to level.dat.

I have noticed this happening for several old worlds (probably related to the old world format) for 1.5.0.