Embedded subtitle files don't work

We have set up our site using Generico to deliver slightly different content to user depending on the device

i) Desktop: Video Player with interactive transcript / subtitles

ii) Mobile: Standard HTML5 video tags + subtitle/track

The videos are MP4 and the subtitles are web vtt (.vtt)

It works well, when the .vtt file is not embedded in the activity. However when embedded, the subtitle file does not cache, or is not fetched or something. In this situation whether the user is offline or online, the subtitle track doesn't show. If the subtitle file is not embedded in the activity it loads and displays correctly.

You can see embedded video + subtitles here (the problem situation)
https://demo.poodll.com/mod/page/view.php?id=201 (50mb video)
https://demo.poodll.com/mod/page/view.php?id=200 (15mb video)

You can see non-embedded video + subtitle versions of the same activities here
https://demo.poodll.com/mod/page/view.php?id=203 (50mb video)
https://demo.poodll.com/mod/page/view.php?id=202 (15 mb video)

To reproduce the problem you will need to a video + subtitle file. You can download them from the above activities if not handy. The HTML5 video tags would look like this:
<video controls preload="auto" width="320" height="240">
<source src="https://demo.poodll.com/somevideos/zombie_problems.mp4" type="video/mp4" />
<track label="subtitles" kind="subtitles" srclang="ja" src="https://demo.poodll.com/somevideos/zombie_problems.vtt" default>
</video>