I think I’m on the good forum for this question :
So I have create a mesh in blender which look fine ( without any invisible faces ) :
The problem is when I import it in Unreal Engine , a face of the mesh become invisible ( but has always his collisions ) :
This face is visible only from inside the mesh , thats the problem :
You have to flip the normal of the “invisible” sides in blender.
In fact, Unreal render only one face of each poly. It’s named “face culling”.
There’s normally no need to render the side of a poly that is not toward the camera 
But in some case, you will want to render the 2 side of a mesh (if you have transparent parts).
To do this, you will check the “2 sided” box in the material you apply on such a mesh, but it’s longer to render such material so don’t use this functionality when you don’t need it ^^.
Try Recalculate Normals in the shading dropdown.
