This is a bit late, but I ran into this exact issue. And since I have 2 F39 boxes, and it worked on one but not the other, it turned out to have nothing to do with installed packages.
I had to remove the gstreamer cache file for gstreamer to pick up I had libav installed:
rm ~/.cache/gstreamer-1.0/registry.x86_64.bin
rm ~/.cache/gstreamer-1.0/registry.i686.bin
Figured I’d put this here for future people stumbling on this!