Yep, that one looks like a close relative of https://bugs.winehq.org/show_bug.cgi?id=9127 (possibly even duplicate).
Last time I checked, Proton-GE didn't include any MPEG video decoder. Probably because it's unnecessary - without Wine-side support, there's no way to access that codec, so there's no point wasting space on it.
Once this thing is merged, that will become a bug on their end. Feel free to report it.
If you want a faster solution, you can take the GStreamer plugin from https://github.com/Alcaro/krkrwine/releases/tag/v1.0.0 and place it in the GST_PLUGIN_SYSTEM_PATH. (I wouldn't recommend installing the rest of that program, it's tested with Proton 8.0 only and may interact weirdly with this MR.)