WEBM VTT subtitles not displayed correctly

When playing a mkv file with embedded vtt subtitles, the raw vtt data is displayed instead of the text. eg

"align:start position:0% what<00:00:07.440> is"

instead of "what is"

Steps to reproduce.

1. Download a video from youtube with auto generated subs:

youtube-dl --write-auto-sub --embed-subs --sub-lang en https://www.youtube.com/watch?v=QaIoW1aL9GE

2. Play the video in vlc 3.0.11 (I tested with Linux and Win10)

3. Enable the subtitles.

Result: The subtitles are displayed like this:

align:start position:0% what<00:00:07.440> is<00:00:08.000> 8-bit<00:00:09.050> search<00:00:10.050> the<00:00:10.290> App<00:00:10.440> Store<00:00:10.830> for

when they should look like this:

what is 8-bit search the App Store for

If I extract the subtitles to a vtt file and use this file, the subtitles are displayed correctly.

1. Extract vtt file:

ffmpeg -i WHAT\ IS\ 8-BIT\ _\ What\ are\ 8-bit\ graphics,\ anyway-QaIoW1aL9GE.mkv -c copy -map 0:s:0 subs.vtt

2. Open mkv file in vlc.

3. Add the subtitle file "subs.vtt"

Result:

Subtitles are displayed correctly.