BSPs are unlit even though directed light is in scene

Hi folks,

I have a scene within a sky sphere. There are a ship and some static meshes in the sphere.
Now I added some new objects by using the BSP box brush.

In the preview I can see the box is lit (shown in the following screenshot):


But as soon as I build the lighting and start the game the box is unlit.


In my scene I use a directional light and a skylight. Both are activated.

Furthermore I have a lighting error, because my sky sphere has wrapping UVs. But I suppose the unlit box is independent of this error?

Anyone have an idea what could cause this issue?

Thanks in advance.

Best regards,