yt-dlp saving as .mkv instead of .webm

I downloaded yt-dlp yesterday and tried it out downloading a few videos, which all came out as .webm. Yet today, for some reason, all the files are downloading as .mkv and always do so unless I put -S "ext:mp4"

No other extensions work.

What can I do to resolve this and make files default download as .webms again?

Edit: It seems some videos download as .webm while others do so as .mkv when just putting the URL in the command. Is there a solution for this or is it just something to live with?