I downloaded an update for the Fire TV app today, and ever since, the Spotify app turns off after 20 mins.

This appears to be because the Fire TV stick doesn't recognise that the Spotify app is running, as it displays stock images/screensaver after 5 minutes, then switches off entirely after 20 minutes. This is what the Fire Stick does when it's not in use while the TV is on, so it's almost like it doesn't "know" the Spotify app is running.

The sound is not affected until the switch-off at 20 mins. This has only happened since the update, so maybe it's something to do with that? The Fire TV app can be a little bit buggy, but nothing like this has happened before.