Got it
That was the issue, however it couldn’t be fixed while in editor. Not even at reimport would it recompute sdfs after i made sure the material is one sided.
Since I had simple planes as walls before I did setup the default material as twosided, but that material would also be applied by default on reimport of meshes which were now thick walls. I switched twosided option on material off but it didn’t do anything until I restarted.
I hope I will be able to in 5.5 finally. As of now not even though m3pro supports hwrt, I cannot use it in unreal.
Anyways, it does look like importing mesh while having twosided enabled by default on the material was indeed causing the issue!
EDIT: I can see now that enabling and disabling the two sided sdf generation in static mesh editor can easily make those meshes hollow and fix them too by disabling it while in editor.
So I assume that option was simply enabled by default on import cause the material had two sided option enabled? Or what esle could cause this to be enabled on import?