Ancient bug, but, I have fixed this upstream with avahi/nss-mdns PR #114, commit 6ecf22a on master. It patches avahi to follow the RFC and never return a scope ID for a non-local address. It is exactly this violation that was causing NFS mounts to fail, as kernel's RPC address parser (rpc_parse_scope_id() in net/sunrpc/addr.c) rejects addresses with scope IDs unless they're local.
The last upstream release tag was v0.15.1 back in 2021 so you may want to backport the fix. Upstream commit: https://github.com/avahi/nss-mdns/commit/6ecf22a PR: https://github.com/avahi/nss-mdns/pull/114
