Subtitles not showing on iOS for MKV with embedded subs

Summary

Playing an MKV with embedded subtitles and enabling one of them doesn't actually show any subtitles on screen.

Minimal project and steps to reproduce

0. Following https://code.videolan.org/videolan/LibVLCSharp/-/blob/3.x/docs/how_do_I_do_X.md#subtitles
1. Play an MKV with embedded subtitles like https://galassia.diegostamigni.com/video/sub-en.mkv
2. List all the available subtitles
3. Enable one
4. No subtitles show on screen

Subtitles are set via:

this.MediaPlayer.SetSpu(trackDescription.Id);

And there are currently no media options specified (font, font-color nor encoding).

Worth mentioning that other than my code, I tried using this sample https://code.videolan.org/mfkl/libvlcsharp-samples/-/tree/master/MediaElement and send that URL in play still with no result. VLC desktop app and iOS app work as expected.

What is the current bug behavior?

No subtitles show on screen.

What is the expected correct behavior?

Subtitles should show on screen.

Does it work on other plaforms? Does it work with the official VLC apps?

I don't know, I can test this on iOS only.

Environment

• OS: iOS
• Version: 13.3
• Device: iPhone device and simulator
• LibVLC version and architecture: 3.1.5.1
• LibVLCSharp version: 3.4.3