There’s a couple issue I’m seeing with that link. The CC tab in your title isn’t showing up originally, until the window is clicked/moved. Once showing I’m seeing the CC is set to “None” instead of “On”, which usually toggles the CC going on and off. For testing this page, would it be a big issue for you to do a clean caption file on a new video object with the default controller?
I add a screenshot of what I’m seeing from the link