I'm having trouble adding subtitles to an AMP Web Story with HTML5 track. The following simple example does not display any subtitles. I've searched for an hour online unsuccessfully. The documentation is sparse. Any ideas?
<amp-story-page id="page1">
<amp-story-grid-layer template="fill">
<amp-video autoplay
width="600" height="800"
layout="responsive">
<source src="https://example.com/test1.mp4" type="video/mp4" />
<track
label="English subtitles"
kind="subtitles"
srclang="en"
src="https://example.com/test1.vtt"
/>
</amp-video>
</amp-story-grid-layer>
<amp-story-grid-layer template="vertical" class="bottom">
<p>Sample text.</p>
</amp-story-grid-layer>
</amp-story-page>