The following probably only serves to add to the mystery at this point.
I have a laptop with built in media keys that is running Debian Stretch and is on spotify 18.104.22.168.gf6bc1b6b-12 30 stable. Unlike my Ubunut Xenial laptop, it is not encountering any problem with the media keys. (The media keys on the Xenial laptop are on a Logitch K750 keyboard and they are still working with snap version 22.214.171.1243.g34a58dea-5 refernced in my prior posts.)
I upgraded to a newer version of the spotify-client this morning. Media keys still don't work. However, they do work as expected for RhythmBox. And, they did work(for several years with no issues) with the linux spotify client.
Here is the version I just upgraded too(via apt):
nick.parry@nparry-laptop1:~$ dpkg -l | grep spotify
ii spotify-client 1:126.96.36.199.gf6bc1b6b-12 amd64 Spotify streaming music client
Let me know if there is any other info that you need from me. Thanks.