Yep, the reliable way to trigger it seems to be try to join a stream in progress, and have the initial connection to livekit fail (which happens to me maybe up to 10% of the time, intermittently). When it rolls over to the backup region, it will never subscribe to the streams. Even if the initial connection didn't subscribe to the streams, which seems to rule out my thought that I just wasn't cleaning up the subscriptions properly on disconnect.
I was hoping to go live with this the week after next, so really keeping my fingers crossed this is just something programming related on my side rather than a bug! :D