Hi @noris,
I just replicated it, and made some additional experiments to narrow it down. To your question: Roon was playing music to the Sonos Zone, which was not grouped with any other zone (never was).
First I started the music in Roon on the Sonos via my MacBook Pro. I then switched on the Bluetooth headphones. They automatically connect to my MacBook. But no music was playing locally (Zone: MacBook Pro). I then turned off the headphones. And the music on the Sonos stopped.
Second I started the music in Roon on the Sonos via my iPhone 11. I then switched on the Bluetooth headphones. They automatically connect to my iPhone, too. But no music was playing locally (Zone: iPhone 11). I then turned off the headphones again. And the music on the Sonos stopped again.
So this happens on MacOS and iOS (both: latest versions).
I then disabled the local Audio Zone in Roon on the MacBook. Started music via MacBook on Sonos zone. Turned the headphone on. And off. Music stopped again.
Next I also disabled the local Audio zone of the iPhone. Started music on Sonos from the iPhone. Turned on headphones. Turned it off. Music stopped again.
Next I disabled Bluetooth on the iPhone. Started music on Sonos via iPhone. Turned on the headphones (so they only connected to the MacBook). Turned them off again. Music stopped playing.
(Ok, I have to admit I did the last experiment too, where I also disabled BT on the MacBook, started music on the Sonos and turned headphones on/off. As expected nothing happened. At least a confirmation that we do not need to send the Ghostbusters…).
I then tried the same experiments playing to a different zone, in this case to my hardwired LUMIN U1 Mini in my main system.
To make it short: It shows the same stopping of the music to the Lumin zone when I turn off the headphone, but only, when the headphone was connected to the MacBook. When the headphone was connected to the iPhone, the playback was immune to the on/off of the headphone.
But - a new strange thing happens: when I play to that Lumin zone, the headphone connected to the MacBook, and I then turn off Bluetooth (instead of turning off the headphone itself), the music also stops. Also here the iPhone behaved differently, when I disabled BT (while headphone connected), the music continued playing. OK.
To conclude: turning off an BT connected headphone on an Apple device / Roon remote can stop the music of an independent, not grouped Sonos audio zone, even if the played music was not started from that device / remote.
This is not related to local audio zones from the remote(s), it also happens when there is/are no local zone(s) on the BT connected device(s).
I suspect there’s a universal iOS & MacOS STOP function, activated when an audio BT device is turned off (or BT is disabled while a device is connected), that gets routed to Roon and wrongly stops (most, see below) playing zones instead of only the zone, where the BT device was connected to.
iOS and MacOS behave differently, depending on the connected endpoint. With Sonos, both OS show that bug, my hardwired LUMIN endpoint was only affected via MacOS.
Hope this helps.