Pushing data as fast as I am, the signal was getting clobbered by itself through reflection. The IC datasheet recommends having series resistors after a certain length between ICs, however since I am nowhere near that, I didn’t bother to put anything in. As it turned out, the data was getting corrupted and adding pull-downs solved it. This is my attempt at a Macgyver-fix. Future revisions will have proper pads and SMD resistors. File this one under lessons learned.