Normally, using the Add Medium button adds the new medium under all existing mediums of the release, but if the release has no mediums yet (only the "placeholder" medium the RE starts with) then the entered tracklist is added to that first medium.

Empty mediums look kinda the same as that placeholder, except for having a checked " I don't know the tracklist for this medium." checkbox - as such, users seem to expect Add Medium to attach a tracklist to the empty medium. While this probably shouldn't be automatic (sometimes you do want to add a second medium to the release), it might make sense to have a checkbox by Add Medium like "Add tracklist to existing empty medium instead" (or something of the sort).

If a release has more than one (empty) medium, we'd need to either just not provide this option or provide a dropdown to select the medium the tracklist should be added to.