Subtitles Vanished After Transcoding: Where Soft Tracks Go
The MKV clearly had subtitles; the converted MP4 does not. Subtitle tracks are treated differently from video and audio in re-encoding. Understand the three forms subtitles take and each one fate.
Three forms, three fates
Subtitles exist in a video package three ways: external files (a standalone SRT next to the video), muxed soft tracks (a separate stream inside MKV/MP4 that players toggle), and hard subtitles (burned into the pixels).
Transcoding treats them completely differently: external files are untouched (separate files); hard subtitles ride the pixels and survive (compressed along with the picture); soft tracks are the fragile one — re-encoding tools process video and audio by default and silently drop subtitle streams they do not explicitly carry.
Recovering a lost soft track
First confirm the source: a track inspection shows whether the original file contains a subtitle stream. If yes, the track was dropped during conversion. The recovery path: extract the subtitle stream from the source file (stream extraction exports SRT/ASS), then decide its next form.
From there, choose by need: burn into the frame for universal display, or re-mux into a container for switchable multi-language playback. Pure remuxing re-encodes nothing and keeps every track — the zero-loss route when the goal is container compatibility.
Prevention: inventory tracks before processing
Multi-track files (MKV above all) deserve a track inventory before any processing: how many video streams, audio tracks, subtitle tracks, and their languages. The inventory decides the plan — keep everything via remux, or extract subtitles first and re-attach after transcoding.
For distribution, remember that a multi-track file completeness only exists inside players that support it — recipients may receive a subtitle-less video you thought was complete. Public sharing favors MP4 with hard captions, or MP4 plus a sidecar SRT.