I looked at your .vtt file and couldn’t find any instance of the character < into it. How did you italicize the subs?
I did that by using <i> as described here: https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API/Web_Video_Text_Tracks_Format#italics_tag_
[edited because itch.io does italic like that too, oops; fixed this message by using HTML entities https://developer.mozilla.org/en-US/docs/Glossary/Entity, in case you need to write here something like <i>]