Hello,
I just checked: the usual browser didn't catch the modified subtitles, but when I created the draft project and uploaded the archive there (with the modified index.html and modified night_of_the_living_dead.vtt files inside), the player on the project page shows the correct modified subtitles.
To edit the index file you can use any text editor (I used Notepad++), the subtitles file is highlighted on the screenshot:
So it could be the in-browser viewing issue.