Sorry for the delay here – I wonder if this is a problem that’s unique to the snap, or if this is something that would affect any mixed Docker+LXC system?
Docker does a lot of hand-holding for making sure networking is “easy” and it does have a tendency to trample the host networking stack (by design), so while this doesn’t necessarily surprise me, it does strike me as odd (since Docker shouldn’t be touching that br0, and should also automatically avoid the IP range it is using by default).