Yeah, that was exactly it. Unifi have a special page in their docs for dealing with Sonos.

https://help.ui.com/hc/en-us/articles/18930473041047-Best-Pr...

I ended up connecting everything with a wired connection and disabling WiFi. Thankfully I have cat6 to every room so it wasn't an inconvenience.

It's worked perfectly since.

How could a Sonos device possibly interact with spanning tree? Are there Sonos devices that act as bridges?