After going past X or Z of 1776 in my world, there are no more chunks, just void. I can place blocks there. I have restarted my client and changed my video settings. I re-made another world with the same seed & settings and it does not experience this problem. But the problem persists, and I find it fascinating.

The world files themselves are too large to attach, but I've put them on a share here: http://1drv.ms/1USAREB