SOLVED: AC3 Mono won't play or transcode unless app is set to downmix to stereo

[coffeetablecandle]

I bet there's something wrong with the container. Just do a straight remux to fix any container errors. Use MKVToolNix or ffmepg.

I think this must be the answer. I used ffmpeg to do a straight copy to a new MKV and the new file plays everywhere without issue. It did set the subtitles to "Default: Yes" but my server automatically sets the default to off, so I'm not sure if that could be the issue. No logs have ever complained about the subtitles either.

I know this is no longer a Jellyfin issue, but do you have any advice for diagnosing the exact issue with the original file? The mystery is really bugging me now.