The reason for “why” is philosophically linked with the true boolean - and thus incomprehensible - nature of the computer being! … or something like that…
Well, no! I have no clue I can only suggest you to re-install the graphics driver… Unless by some reason, only the older drivers behaved decently, and these don’t… hmm, did you upgrade anything? DirectX or something like that?
Are you running on a laptop or desktop PC ? (PCs tend to be more fixable with driver updates than laptops, I think)
But a “driver issue” is the most plausible thing that could be happening…
Still, can you tell us what world sizes your are using, and preview size an such? Thanks!