Maxscale( 2.1.7) no response to query

we are using the maxscale 2.1.7， but our java application execute a read-only no transaction query ocatinally hang-up to timeout until java application get the exception("SQLSTATE(08S01), Communication link failure"). the wait time is just equal to the maxscale readwritesplit serivce connection_timeout.
I have tcpdump the network traffic. it seems the java application send the mysql query to maxscale. but maxscale do not response the query!

Serivce Config
type=service
router=readwritesplit
servers=server1,server2,server3
user=root
passwd=xxx
max_slave-connections=100%
use_sql_variables_in=master
max_slave_replication_lag=1
router_options=master_accept_reads=true,rwsplit_safe_interval=50
connection_timeout=120