This started a couple of months ago, but it took me a while to work out what was causing it and pinning it down to the Companion app.
If I open the Companion app whilst riding and using Spotify to control music playing on another device, after the end of the currently playing track you can no longer access the Spotify controls by pulling down from the top of the screen to open the notification centre.
Without the Companion app running Spotify works as expected and is available on the lock screen and notification centre.
It’s done this with both iOS 11 and 12 on two different phones. Zwift and Spotify are running on the same Windows 10 computer.
It doesn’t appear to be an issue if I open the Companion app whilst not riding.