Ben, please consider supporting Mario (https://www.psychtoolbox.net/) by buying priority support.
Be careful with a letterbox around the video, as depending with pixelformat settings these may come out just wrong when encoded and thus be visible against your background.
Given what Mario says, it may be worth trying to force hardware decoding on or off. I bet there is some GStreamer magic incantation to force either, if possible, but i do not know of it. Trying a different encoder (ffmpeg for instance provides multiple for H.264) may be worth experimenting with either. You’re “lucky” its always the same videos, as that at least makes debugging tractable.