Once again, multimedia keys are not working globally. They work in individual applications, but only when you're in those applications. If you're playing music in a media or music player (i.e. Banshee, VLC, MPV) you can use the multimedia keys to control playback in those applications, but when you move to a different application, you cannot use the multimedia keys to control playback of media playing in a media player unless you return to the player - but the multimedia keys cannot control the playback on a media player outside of said media player. I know this is kind of a repeat of Bug #13848 but I thought I'd bring it up again as it seems it might not have been resolved back then, or it has reappeared.
Have you tried the option "Enable multimedia keys for playback control" introduced in xfce4-pulseaudio-plugin 0.3.3? See: https://bluesabre.org/2017/11/27/development-release-xfce-pulseaudio-plugin-0-3-3/
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