Avidemux could improve how it manages subtitle tracks. When I try to append one MKV file to another MKV, the subtitle tracks are missing from the output MKV.
Not that support for soft subtitles is deemed insignificant or is not wanted, but https://avidemux.org/smif/index.php?topic=19128.0 (https://avidemux.org/smif/index.php?topic=19128.0) still applies.