Faces can only be visible from one side. Maybe that's the problem.
I think what your seeing when you rotate is the a visible face of that piece through the invisible face..That model section is prolly' inside-out or something since faces can only be seen in Blockland from one side...
Flip triangles by pressing it in blender.
Huh? Also, when it spins around there is no problem.. I'm starting to think I found the magical angle where the Blockland lighting system lights the face so it's the same color as the faces behind it. The handle is lighter than the rest of the box.
Yeah it could be that those parts of the handle are being lit in such a way that they are the same colour as the box. Try tinting the handle very slightly with a cold colour like blue; something that's not immediately noticeable.
Try pressing ctrl a to apply better lighting than joining a cube.
It was the problem I described a few posts up. I just scaled the handle a bit and it's fine now.