UPDATE:
So, My hard drive crashed. I've re-installed most of my programs now, and somehow, the issue has been resolved.
What the issue was, however, I'm not sure. So for the record's sake, here are the most notable changes:
There are much fewer programs now, due to fresh install: Possible conflict caused issue
May have been issues with the original Substance Install: Reinstall and see.
(Most likely) It's possible the tablet may have shipped with the wrong install software/drivers. I downloaded them from Huion's site, and I've noticed the UI is different in a few subtle ways.: Double check tablet drivers/software.
I had made sure that the driver was up to date, but it may have been I'd just been given the wrong driver.