Hello there @AdamGaming_CZ !
Checking around the community, another thread had a similar issue to yours:
From their findings, combining mesh distance fields with two sided materials will result in the effect you are seeing. The solution should be to disable Two Sided in your wall material, make sure you hit “Apply” on the changes, then force regeneration of the distance fields, either by making a small change on the mesh, or re-importing it.
As for further checks after that change is done, if your scene is using Nanite, disable it. After that, clear your shader cache, via closing UE, and deleting folder DerivedDataCache at …\AppData\Local\UnrealEngine\Common. Restart the engine, and allow it to rebuild.
You should also ensure that your wall’s geo is not compromised, even a simple mesh can cause a lot of problems if it has inverted normals or non-manifold geo. Simply give a pass in Blender or another 3D tool of your choice to make sure of that.