I would swap the speakers over temporarily, just to see if it’s the speaker, or the stand itself. Maybe make a short mobile video of the issue, if practicable …and then contact either Sanus, or Sonos, customer support (as applicable).
Here is the link to contact Sonos CS:
https://support.sonos.com/s/contact
Here is the link to contact Sanus CS:
https://www.sanus.com/en_GB/support/
You might try adding some foam rubber strips under the stand to see if it is an issue with it making good contact with the floor.
Holding the stand probably dampens the resonance.
While no longer considered the environmentally safest approach, speaker stands were once commonly filled with lead shot.
You could experiment with inserting or enclosing the stand with rubber tubing.