the subtitle list is empty

[fds39]

I wanted to download the subtitles of a video; at first I thought they were unavailable because the list was “empty”.
So I downloaded them manually from the command line. I didn't know the .vtt subtitle format.
Then I found that the subtitles could also be downloaded with yt-dlp GUI in the “empty list”.

So there are some small errors to correct. But why is the format different? srt vs vtt
I guess it's a conversion... is it really necessary?

https://www.arte.tv/fr/videos/121691-001-A/guerre-et-paix-1-4/