In this case even if your original file has 2 video tracks, 4 audios, mpeg2 video codec, or whatever, the output will always have the correct video and audio format and it will discard any other unwanted streams. But you can tranform it: ffmpeg -i input.mkv -vcodec copy -acodec copy -scodec movtext output.mp4 This way you take all streams including the converted subtitles. The best way to handle this would be to map the streams you need from the original file -map 0:v:0 (first video stream) -map 0:a:0 (first audio stream) and then use the codec selection to transcode into -c:v libx264 -c:a aac for MKV containers: copy, ass, srt, ssa for MP4 containers: copy, movtext for MOV containers: copy, movtext. You may not use a mp4 container if the subtitle stream contains a format other than the special MPEG-4 Timed Text format.

You were lucky that previously the video format on the original container was compatible with the mp4 container, but as you noticed it did not work this time.

The root cause of the problem is the -c copy command, what it does is that it actually copy everything inside the original file and mux it to a new container, in your case mp4 and it also is copying all the other stuff (subtitles etc).

Ffmpeg mkv to mp4 no audio reddit