[Sivar]

Admittedly, I haven't tried a single-thread encode again because they take a bit of time to complete. Aside from a small probability that single-thread encodes are unrelated to the other problems I ran into, it appears so far that my x265 issues were a combination of:
1. An old version of ffmpeg
2. Two bad source videos. The sources had graphical corruption in one spot roughly midway through the video.

I've never experienced either until my first look into x265, and I've transcoded a lot of media and have spend hundreds of hours researching, programming for, and tweaking my transcoding hobby. I thought that gave me the right to commit the cardinal sin of assuming that correlation implied causation. My apologies to the x265 team.