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I'm pretty sure that's because two UV co-ordinates have the same position on the triangular faces, which causes problems with the face being hidden when adjacent to another brick of equal or greater area. There's not much I can do about that.
Update: I should have all of the terrain bricks done by this week! Brick will now use zones, will come in only two separate packs and will allow building on ramps. There are some draw-backs, but it should be worth it. Also, I'm working on other brick packs to supplement the removal of interiors. I attached a preview below (note: normals have not been set up yet on the bricks).