WebVTT Subtitles broken when progressive loading is enabled #7648

What version of Hls.js are you using?

1.6.14

What browser (including version) are you using?

Chrome 142.0.7444.162 (Official Build) (arm64)

What OS (including version) are you using?

MacOS Sequoia 15.7.1

Test stream

https://hlsjs.video-dev.org/demo/?src=https%3A%2F%2Fstorage.googleapis.com%2Fshaka-demo-assets%2Fangel-one-hls%2Fhls.m3u8&demoConfig=eyJlbmFibGVTdHJlYW1pbmciOnRydWUsImF1dG9SZWNvdmVyRXJyb3IiOnRydWUsInN0b3BPblN0YWxsIjpmYWxzZSwiZHVtcGZNUDQiOmZhbHNlLCJsZXZlbENhcHBpbmciOi0xLCJsaW1pdE1ldHJpY3MiOi0xfQ==

Configuration

{
  "debug": true,
  "progressive": true
}

Additional player setup steps

No response

Checklist

• The issue observed is not already reported by searching on Github under https://github.com/video-dev/hls.js/issues
• The issue occurs in the stable client (latest release) on https://hlsjs.video-dev.org/demo and not just on my page
• The issue occurs in the latest client (main branch) on https://hlsjs-dev.video-dev.org/demo and not just on my page
• The stream has correct Access-Control-Allow-Origin headers (CORS)
• There are no network errors such as 404s in the browser console when trying to play the stream

Steps to reproduce

1. Enable progressive in HLSConfig
2. Start Stream with WebVTT Subtitles
3. Start Playback
4. Enable WebVTT Subtitles
5. Notice WebVTT Subtitles never load

Expected behaviour

Subtitles should not be loaded progressively. Subtitles should load and render to the user.

What actually happened?

Subtitles never render.

Console output

N/A

Chrome media internals output