Interesting. That doesn't seem to be specified in the RFC1122 standard or the Li...

Interesting. That doesn't seem to be specified in the RFC1122 standard or the Linux ip(7) docs, but it's an explicit special case in the kernel (ip_route_output_key_hash_rcu):

        if (!fl4->daddr) {
                fl4->daddr = fl4->saddr;
                if (!fl4->daddr)
                        fl4->daddr = fl4->saddr = htonl(INADDR_LOOPBACK);
                ...