The past two times i've tried to make a enclosed space, it ends up looking like this. https://gyazo.com/e765604d1a30b48f4667f0edfea71311
It is as if it has a low opacity since you can kinda see through it.
https://gyazo.com/e6242a859d808c79742c0bb560d3e4f6

try seeing if the textures are patch/decal textures because those textures are slightly clear like how the walls are in your map

When using textures, you need to use the ones that say "lit" not "decal".
That is clearly a decal that complements its "lit" counterpart.

Each texture has a texture type in red under the texture name. Try applying a texture with 'lit' under it and see if it still happens.