Comment 9 for bug 1841621

Part of the reason this behavior exists in Avahi is that many applications do not correctly retrieve the scope ID (interface index) when doing hostname resolution, and if not supplied then connection to such a link local address will fail. Applications are likely to receive such an address at random.

Also until very recently nss-mdns didn't actually support passing through that scope ID, though it now does in the latest versions.

So changing Avahi to always return these link local IPs is much more likely to break pretty much every other application except Pidgin. To my mind what Pidgin should do to resolve this issue is to explicitly either not block based on the incoming IP address, or, bind explicitly to the IP address that is being advertised to prevent the connection being sourced from the link local address.