DNS fails when Vodafone Power Station is offline and powered on

[nagaraja]

Hello,

I have a working triple wans configuration. One of these is through a Vodafone Power Station as router (https://www.vodafone.it/portal/Privati/Supporto/Fibra--ADSL-e-Telefono/Installare-e-configurare/Vodafone-Power-Station). I use DNS resolver to forward client queries from DC and proxy. I noticed a strange behaviour when router go offline using nslookup on DNS resolver, results change from:

www.google.com
Server: server.local.lan
Address: xxx.xxx.xxx.xxx

Risposta da un server non autorevole:
Nome: www.google.com
Addresses: 2a00:1450:4002:80a::2004
216.58.205.164

to

www.google.com
Server: server.local.lan
Address: xxx.xxx.xxx.xxx

Risposta da un server non autorevole:
Nome: www.google.com.local.lan
Addresses: yyy.yyy.yyy.yyy

Where name is @local.lan (my local domain) and yyy.yyy.yyy.yyy (my router internal ip).

Of course in this case i have connectivity using other wans but my DNS always fail

Why do this happen?

thanks anyone