This is a bug I have personally reported to both GSUS and GSBUD. It seems to happen mostly in a parallel projection and lines appear randomly between windows. Your example is quite mild compared to others I have seen (if that's any consolation - which I'm sure it's not!
)
Other problems include no shading to the underside of roofs, and lines between walls (and slabs) that have exactly the same materials.
Unfortunately these are not fixed in 2256 but keep your fingers crossed for future releases!
Cheers,
Link.