DNS responds by 2 addresse VIP

This usually means that the response has fallen through to bind (either the local bind, or to another DNS server behind the GTM), and that the GTM process did not provide the response.

For example, if all GTM pool members are down, and the pool lb methods include 'Return to DNS', then the request would be passed to bind, which has no knowledge of the state of the pool members, so just returns what is configured.

Have a look at the statistics for the wideip (DNS / gslb / wide ips / / Statistics - wide ip) - that will show you counters indicating how the query was handled.