thats because you are using bsp for your level design. dont.
Get blender or another 3d program and model your structures with that.
ue4’s bsp tools are not made for modelling content.
BSP is an FIFO or order-of-operations driven substance, meaning brushes added copied or replaced can be hidden or overridden by others that follow. the order can be adjusted. In the brush details the is a section to bring brushes forward or back, this may help alleviate your problem.
that being said, i do partially agree with Luos, BSP is good only for block out phase, simple volumes, and the most basic of shapes and assets… It should not be used to build Meshes and certainly not an ideal tool for making game ready structure; you would be better off using a modeling program , such as Blender(free) Maya, or 3ds Max to do your mesh work in.