Levels window - empty list

I’m trying to use the world composition feature, but I can’t seem to add any levels to my current project.

I’ve enabled world composition feature, went to the levels window, but I only see
“Persistent Level”. I’ve tried creating new level, and it creates a new level, but it’s not in the list (it’s in content browser).
I tried dragging the level from the content to the window, but it doesn’t do anything (it shows “Add Level” when hovering the window).

What am I missing?

I started with an empty project, then created landscape and models,
(saved all data) and then enabled world composition feature.