I don’t use that MSP (so I’m not entirely sure here), but I think Spotify do not allow more than one audio stream at a time for their premium account users - it’s their limitation, I believe. See these links:

https://www.drmare.com/spotify-music/play-spotify-on-multiple-devices.html

https://community.spotify.com/t5/Subscriptions/Stream-in-more-than-one-place/td-p/4208098

I know that multiple audio streams are allowed via Deezer HiFi and Amazon Music Unlimited, but I’ve not used Spotify (other than it’s free version) to actually confirm that they limit their service to just one stream, but if it is allowed, you will need to start any additional streams via the Sonos App and not using voice control etc; (which has its own stream-limitations too).

I thought that was the case, but I just tested, and I have three different rooms all playing Spotify.  I do have a family plan, but all of them are playing off only one account under that family plan.

Now opening Spotify and playing a song on my iPad kills the Sonos stream immediately.

If things are as they were when I last used Spotify, you can play multiple streams on Sonos if you use the Sonos app but only one stream if you use Alexa / Google Home. This is consistent with what @Ken_Griffiths said.

In effect, Spotify regards Sonos as a single stream if the Sonos app is used. An iPad stream would count as a separate stream.