I get this message spawned every 10 seconds on all terminals.

multicastBeacon: Failed service 'Service@0xc00014ebc0 created by multicastBeacon@0xc0002025a0/writer' (2.012610 failures of 2.000000), restarting: false, error: write udp [::]:36056->[ff12::8384]:21027: sendmsg: cannot assign requested address

why is a lapalie treated so aggressively?

This is not the first time I see aggressiveness here.

It is understandable that people want to disable UDP and tcp6. Spamming them with aggressive kernel messages is not OK.

You can disable ipv6 if you want, but then maybe disable the corresponding addresses in Syncthing. I don’t know why they would end up on all terminals, we print errors to stdout.