Most of their devices act as bridges; some of the newer ones don't. Some have multiple ethernet ports, and anything that has both an ethernet port and is part of their "sonosnet" mutant-wifi will bridge between sonosnet and their ethernet port(s) with spanning tree using classic (pre-RSTP) link costs.
If you're not careful you can end up with the "best" path between two switches going over the sonos-to-sonos wifi.
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.