WebVTT subtitles track in a WebM/Matroska file is unsupported

According to this issue from MKVToolNix, the ways a WebVTT track is stored in a WebM or Matroska file are different.

With MKVToolNix, created a Matroska file with A/V tracks compliant with the WebM format and a WebVTT track.

With FFmpeg, created a WebM file from this Matroska file:

$ ffmpeg -i video.mkv -c:a copy -c:v copy -c:s webvtt video.webm

Installed totem's flatpak with:

$ flatpak install flathub org.gnome.Totem

When trying to play any of these 2 video files with totem, the following error is spit:

Missing plugin: gstreamer|1.0|totem|application/x-subtitle-unknown decoder|decoder-application/x-subtitle-unknown (application/x-subtitle-unknown decoder)