Spotify stops music (often when I open new apps or when I get a notification)

Welcome to the forum.

Your problem may be related to this:

and in particular post 36:

However, these were reported before the update to A15.

I presume the previous instance of the problem was resolved by an update and is now back again.

Make sure that Spotify app is allowed to run in the background:
Settings > Apps > All apps > Spotify > App battery usage > Allow background usage needs to be enabled.

If that doesn’t help, I advise you to file a bug report on support.fairphone.com.