I was having trouble using my Media Keys on cosmic and then installed the playerctl package and it worked. Not sure if you want to make it a required prerequisite.
Indeed, it is use here https://github.com/pop-os/cosmic-settings-daemon/blob/2f17f33875315a4cf463f82c5dceca4d83a75bfd/data/system_actions.ron#L23
I believe this is fixed. Please comment if it is still a problem!
Metadata Update from @ryanabx: - Issue status updated to: Closed (was: Open)