UE 5.2 Lighting Issue

Hi. I have a lighting issue that does not happen in 4.26. I cannot get a hollowed out geo cube to have a completly dark centre during gameplay. When the game starts, it is dark, then the light starts to get brighter until every wall is visible within 3 seconds. If I look down towards the floor, the scene go completly dark again and if I look up the 3 second cycle of showing the light occurs again.

I have only been using unreal solidly for months now. I am unsure if this is some setting I cannot find or if its a bug. I have tried building a house: 4 BP walls and a BP roof, all with materials and I get the same issue. I am only using the BP meshs as a testing platform.

Anyone any ideas?

My version of unreal is 5.0.0-16682836

Try making the walls of the mesh independent.

Here is how to debug your lighting: Lumen GI is screen space? - #8 by pezzott1

Hope it helps