Since the last update of Spotify (revision 22 - version 1.0.92.390.g2ce5ec7d-18) the multimedia keys do not work.
The only modification that I’ve done is this: $ sudo mv /var/lib/snapd/desktop/applications/spotify_spotify.desktop /var/lib/snapd/desktop/applications/spotify.desktop
In order to make the it available in the sound applet. I’ve been doing this for the last 3 or 4 revisions without problems.
I have spotify revision 22 on Ubuntu 18.04. The media key (I only have one) works here. I can play/pause just fine with no changes to the desktop file.
alan@hal:~$ snap version
snap 2.35.4
snapd 2.35.4
series 16
ubuntu 18.04
kernel 4.15.0-36-generic
The keys also work OK for me on Ubuntu 18.04…will try to remove it and install it again to see if something related to the changes done to the desktop file.