Drupal 8 Image Styles Not Generating - LEMP on CentOS 7
I finally found a solution to this problem! As it turns out, systemd is configured to give php-fpm a private /tmp. I had to edit the .service file to set PrivateTmp=false. It is worth noting that if you're running multiple sites on of the same server, PrivateTmp should be set to =true for security reasons.

For reverence, here is the thread that saved my rump in the end.

Thanks again.