Weird rendering while loading a world composition map

I have a map with world composition enabled and 1600 sublevels. It takes some seconds to open the map. While loading there is a weird rendering. It appears in “Standalone Game” and the packaged release build but not in PIE.

I made a video of the rendering. The throbber on the bottom right is not made by me, so this has to be a engine thing.

269775-ezgif-3-8af407dab69a.gif

I narrowed it down to appearing after the underlying LevelScriptActor::constructor and disappearing before the Level::BeginPlay call. It seems to be a rendering of the persistent level with initial values (Player at 0/0/0; CameraBoom arm length at 0 etc.) and no streamed in sublevels.

What can I do to prevent this to happen or is this a bug?