Problem with extracting subs in WEBVTT format

Good evening, everyone, my problem is the following and the truth is that I have tried everything. I have an MKV created with the “Seal” app, which is excellent for downloading YouTube videos for Android because it can generate an MKV file with the subtitles it generates. youtube automatically to any language the problem is that when I want to extract that subs to edit it it always throws an error with any software that is used to extract things from a mkv, I already tried it and it doesn’t work, the truth is, then use MKVTOOLNIX to extract the subtitle as an MKS file And if I manage to extract the subs but the file comes with many internal errors and leftover text, that is, a total mess, COULD YOU HELP ME?