no_default_search_domain does not take effect

no_default_search_domain in dns_resolver_options does not take effect

*Description*:
I am using dynamic_forward_proxy cluster, and sub_clusters_config is selected. To suppress too many request towards local DNS, I configured DNS resolver in cluster level

But the test result is that envoy still search the default domain, and dns resolution failed in my case.
my host name in request is: a.com
my expectation is envoy query a.com directly to my configured dns server
the result is envoy query a.com.mynamespace.svc.cluster.local, mynamespace.svc.cluster.local is the first records in my /etc/resolv.conf

in my /etc/resolv.conf:
search mynamespace.svc.cluster.local svc.cluster.local cluster.local abcd
nameserver 1.1.1.1
options ndots:5