MPV-0.29.1 in that repo builds against that system ffmpeg 4.1.1 instead, the way God and Debian intended. I put ffmpeg 4 in its own separate repo, since a lot of applications in the main OBS repo won't build against it-putting it there would break them. It's a non-Debian style package, but I had to do that for mpv>= 0.28 Kodi is using the ffmpeg 3.4 libraries in my main OBS multimedia repo.
That particular version of mpv 0.28.0 builds and uses its own internal version of ffmpeg 4, which is why you're seeing those newer libraries for it only.
