Well, I've been working on the workaround suggested by various posts with limited success, but I think this may be the ticket until they get the gremlins figured out.  

Setup where I had the problem: Firefox (also Chromium) on Linux Lite

I login using Facebook.

Workarounds: Firefox User-Agent Switcher [OR] Chrome User-Agent Switcher set to Microsoft Edge

See attached screenshot.

The last row in the panel UI is dedicated for the control options including a button (at the lower left) to change the UA to its default value.

Update: I found this info. You can specify a particular URL or domain so that UA is only applied to that URL or domain (read FAQ for detailed description).

For this Spotify issue: in the box at the bottom where it says all_urls, replace that with spotify.com. The extension/addon remembers it for future broswer sessions.

Firefox User-Agent Switcher

Chrome User-Agent Switcher