OK, so I tried avidemux, but it has the same issue as before where everything works perfectly until you get to the exact moment the clips are joined, and then it freezes on the last frame of the last shot before the switch and the image gets all pixelated and just screwed up. But if you drag your cursor past this point before you get there it’s fine. (Tested with both QuickTime and VLC.) It only happens if you watch at the exact point of the splice. This issue was even there when I tried manually concatenating the clips with ffmpeg, so it must be something about the clips not matching up right or something. Perhaps because they were encoded differently? Don’t know.