I tried this also, but for some reason I need to have the focus on Spotify's window in order to work. Strange enough the Previous button doés work without having focus on Spotify. I've added 4 custom commands using the keyboard settings, reassigning the already existing bindings for those media keys.

When I use xbindkeys, I don't have this issue 😕

ps. Here are the config lines for xbindkeys, including next/prev