Gstreamer plugins missing for Bad Rats

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Game information
Bad Rats

Distribution name and version where applicable
Fedora 40 (Silverblue)

Flatpak info
Flatpak 1.15.8
default
host

Problem description
When trying to run the game, all that can be seen is the Gstreamer TV test screen, with the original graphics flickering in and out occasionally.

Proton logs show that a Gstreamer plugin is missing:

winegstreamer error: decodebin0: Your GStreamer installation is missing a plug-in.

I have installed org.freedesktop.Platform.ffmpeg-full and org.freedesktop.Platform.ffmpeg_full.i386 from Flathub and this issue persists.

Additionally, the game does not display unless using Wine Virtual Desktop, but I don't know if that's a Flatpak-specific issue.

Does this issue reproduce with native Steam
No. This issue does not happen when using native Steam.