Todos os sites que não mostram endereço ipv6 terão esse problema. Pode ser que a troca de pilha não esteja em execução no IPV6:
root@ijaz-HP-EliteBook-8560p:~# host www.google.com
www.google.com has address 216.58.210.68
www.google.com has IPv6 address 2a00:1450:4009:811::2004
root@ijaz-HP-EliteBook-8560p:~# host www.ncp.edu.pk
www.ncp.edu.pk has address 111.68.96.165
www.ncp.edu.pk has IPv6 address 2400:fc00:854a:aaaa:700d:d110:e593:8d00
root@ijaz-HP-EliteBook-8560p:~# host stackexchange.com
stackexchange.com has address 104.16.116.182
stackexchange.com has address 104.16.119.182
stackexchange.com has address 104.16.118.182
stackexchange.com has address 104.16.115.182
stackexchange.com has address 104.16.117.182
porque não está mostrando nenhum endereço IPv6.