There’s a ton of issues there
First off–you shouldn’t construct flat surfaces like walls out of multiple pieces, there’s no benefit of doing that, in UE4 you will get lighting issues because each object will be separate, but also you will have worse performance because each object will increase the number of draw calls. It also increases the number of polygons since you could do the whole wall in one quad otherwise.
Another issue is that you’ve got a bunch of faces that won’t be visible, the top and bottom caps and the back side which contribute nothing and just take up space, also potentially causing visual problems.
For the walls/floor/ceiling you need to have the corners line up exactly and not extend beyond each other. Otherwise, the lighting or shadow on the non-visible areas will extend inside to the visible areas.