Might be worth sending this feedback to Spotify as well, since it is their app you’re using. That way, both companies can investigate where the issue lies. 

The issue happens also if I’m using the Sonos app

I have raised the issue with Spotify support:

Shakay G

 at 21:45, Nov 8:

This has been something that was pointed out to us recently by some users.
Our tech folks actually have been checking out this issue. We'll get back to you by email when we have an update.

Ah, that certainly wasn’t clear from your original post. 

When you power cycled everything, were the Sonos speakers powered down while you power cycled the router? If not, give that a shot, since what you’re describing now is less about the app, and much more about issues with your speakers staying connected to the router. I’m guessing a duplicate IP address issue, which would not be cleared by sequential power cycling. 

If that doesn’t work, perhaps you might submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, or post the number here.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, there are more options available beyond just the diagnostic analysis. 

Help desk was very helpful, added:

We have tried to have it fixed recently but we started working backstage more with Sonos to try and find an exact reason as to why this has been happening.

Yes, switched EVERYTHING off, left it off for a minute or so, then powered up again.

Thanks, worth a shot!

My confirmation number is: 1016644386