I built gstreamer-plugins-bad 1.16.3 in my home project giuschwim. May be someone can have a look, because I am by no means an OBS expert. An interim solution might be simply to link Essentials/gstreamer-plugins-bad to openSUSE.org:SUSE:SLE-15-SP3:Update instead of openSUSE.org:openSUSE:Leap:15.3:Update.