Do you think that you might have some flipped faces? Try flipping them in your 3D package and re-exporting. Also, make sure that the origin (pivot) for the object is exported correctly. If you give me a little more detail about the model I could help more.
It looks like you have some edges crossing in the BSP brush. Un-crossing them and re-render. I also suggest using a 3D modeling package instead of BSP brushes. Hope this works:)
I designed my house in Unreal Engine 4 Version 4.17 but after lighting rebuild a few meshes disappears.
The one picture is showing the mesh that are disappeared.
And the other one is showing where the meshes are still there.
(Sorry for bad english)
Are you using a double sided material by any chance? Would also love to see the normals on the meshes that disappear. Or are you even using Unreals BSP?
Im using the “M_Basic_Wall” material from starter content.
Yes it is BSP, the box mesh and I edited it to a triangle.
I recommend to not use BSP anyway to model an interior and if you want to bake lighting, but for a fast solution you can try to create a static mesh from your triangle and see if that solves the problem. You can find the “Create Static Mesh” button when you click on your Triangle Shape, Details, Brush Settings, then make sure to expand the settings clicking on the triangle.
All of your anwsers helped me to fix that, thank you a lot! 
I did all of the solutions and it worked
Thank you, it worked!
Thank you!
Thank you, it worked now