Forge? Blocks won't stay put

Hey guys,

Been playing with the halo 5 forge and when I make a floor using lets say 9 blocks (64x64x64 Simple) to form a square platform they’re level and perfect in the forge but the moment I play the map in custom game they’re on the the piss and not lined up.

Go back into forge to check them and they’re fine. So my question is, Does this happen often and can I fix it or is it just bad luck?