I get this exact problem, but only when using optifine. I've tried C3, D4, and D5, with multi-threaded loading both enabled and disabled, and without fail, I get either a big stripe of chunks missing, or chunks just stop appearing a few hundred blocks from where I load in to. I've tried new worlds, tried all the optifine options I can imagine, but the problem persists. As soon as I remove Optifine, I can load in and they appear just fine, so it has to be an optifine problem, right? I'm using the latest FTB Ultimate, and the Forge version is like 534 or something, so it's not the outdated one that's incompatible. Not sure what to do from here??