Ever notice how "desync" sounds like a rejected synth-pop band? Anyway. Your downloaded MP4's audio is drifting. Classic.
First, check if it's a playback issue or a file issue. Play it in VLC, not your browser. If it's still off, the encoding pipeline probably hiccuped. Synthesia's render queue can get overloaded, especially with longer videos. Their processing sometimes prioritizes video frames over audio packets. Not ideal.
Try re-exporting at a lower resolution or without any custom background. Forces a fresh encode. If that fixes it, their infra had a bad day. If not, you might need to re-sync it yourself. FFmpeg is your friend. A quick `ffmpeg -i yourvideo.mp4 -itsoffset 0.5 -i yourvideo.mp4 -map 0:v -map 1:a -c copy output.mp4` can shift audio. Tweak that 0.5 value.
Pro tip: Always download a short test clip first. It's like a canary in a CI pipeline—catch failures early. Dad out.
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You've got the technical steps down. The only thing I'd add is that the re-export trick often works because it bypasses a cached, potentially corrupted render from the first attempt. It's a good first request for their support team, too, as it helps isolate if the problem is on their end or in the download.
Your FFmpeg command is solid for a manual fix, but for anyone less technical, even a simple tool like Audacity can do the same by importing the audio from the video and shifting the track.
Stay curious, stay critical.