Have you submitted a diagnostic, and contacted Sonos support directly? You mention “as I wrote earlier” but since I don’t know where that was, I don’t know the history of your interaction, beyond this post. 

While the community can often help in many situations, we are hampered by the fact that we don’t have access to the hard data of a diagnostic, which can sometimes point to a specific issue that isn’t as obvious from an anecdotal report. And since this is a community site, not a direct contact with Sonos support, sometimes information can get ‘lost in translation’ between the forum moderators, good people that they are, but few, and the actual Sonos support folk.   

@Airgetlam  - Thanks for the quick reply. I will get in touch with Sonos support regarding this. I tried a search, but nothing relevant came up, so I posted here. BTW “as I wrote earlier” refers to my opening line in the same post...

Ah, my misunderstanding, apologies. 

It’s hard to tell, based on what you wrote, whether this is a network issue, or an actual software issue, or even if it’s something on Spotify’s side of the implementation. A diagnostic, and discussion directly with Sonos support would likely go a long way to identifying exactly where the issue is, and I would hope provide a useful solution. 

2yrs ago you also wrote this:

On both of the "Living Room" stereo paired speakers, the control buttons on the speakers are unresponsive. They just do not work. Neither the volume buttons, nor the pause/play/skip button. This is incredibly annoying as those are the most accessible ones. On all of the other speakers, the buttons work just fine and the sound is fine everywhere

Now here's the weird part. The Living Room pair was a WHITE set (with non-functioning buttons), and the Dining room was a BLACK set. I decided that I liked the BLACK ones in the Living Room, so I swapped them and renamed them using the app. NOW, it's the BLACK ones that the buttons stopped functioning on! IT ALWAYS APPEARS TO BE THE "Living Room" SET WHERE THE BUTTONS DON'T FUNCTION. 

I assume the Playbar/surrounds are now in your ‘Living Room”.

This suggests to me that there are connection issues in your living room.

Is your Living Room far from the router? If so, do you have more success with Spotify if you first try connecting to a another speaker closer to your router, then attempt to group the Playbar? Also are any of your Sonos speakers wired to your router? If not, are you able to wire one temporarily to see if that improves your experience with Spotify?