hint "maxscale route to slave" does not work

I have a mysql master with 2 slave

=================my configure file begin
[maxscale]
threads=auto
log_info=1
log_debug=1

[server1]
type=server
address=10.40.12.101
port=3366
protocol=MariaDBBackend

[server2]
type=server
address=10.40.12.102
port=3366
protocol=MariaDBBackend

[server3]
type=server
address=10.40.12.103
port=3366
protocol=MariaDBBackend

[MariaDB-Monitor]
type=monitor
module=mariadbmon
servers=server1,server2,server3
user=lbadmin
passwd=lbadmin
monitor_interval=2000

[Read-Only-Service]
type=service
router=readconnroute
servers=server1,server2,server3
user=lbadmin
passwd=lbadmin
router_options=slave,master

[Read-Write-Service]
type=service
router=readwritesplit
servers=server1,server2,server3
user=lbadmin
passwd=lbadmin
router_options=master_accept_reads=true
filters=Hint

[Hint]
type=filter
module=hintfilter

[MaxAdmin-Service]
type=service
router=cli
[Read-Only-Listener]
type=listener
service=Read-Only-Service
protocol=MariaDBClient
port=4008

[Read-Write-Listener]
type=listener
service=Read-Write-Service
protocol=MariaDBClient
port=4006

[MaxAdmin-Listener]
type=listener
service=MaxAdmin-Service
protocol=maxscaled
socket=default

=================my configure file end

1. maxctrl list servers
   ┌─────────┬──────────────┬──────┬─────────────┬─────────────────┬──────┐
   │ Server │ Address │ Port │ Connections │ State │ GTID │
   ├─────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
   │ server1 │ 10.40.12.101 │ 3366 │ 0 │ Master, Running │ │
   ├─────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
   │ server2 │ 10.40.12.102 │ 3366 │ 0 │ Slave, Running │ │
   ├─────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
   │ server3 │ 10.40.12.103 │ 3366 │ 0 │ Slave, Running │ │
   └─────────┴──────────────┴──────┴─────────────┴─────────────────┴──────┘

hint "maxscale route to master" or "maxscale route to server server[1-3]" works fine
but hint "maxscale route to slave" does not work

=============debug log begin

2019-01-11 13:59:43 info : [readwritesplit] Servers and router connection counts:
2019-01-11 13:59:43 info : [readwritesplit] current operations : 0 in [10.40.12.101]:3366 RUNNING MASTER
2019-01-11 13:59:43 info : [readwritesplit] current operations : 0 in [10.40.12.102]:3366 RUNNING SLAVE
2019-01-11 13:59:43 info : [readwritesplit] current operations : 0 in [10.40.12.103]:3366 RUNNING SLAVE
2019-01-11 13:59:43 info : [readwritesplit] Selected RUNNING MASTER in [10.40.12.101]:3366
2019-01-11 13:59:43 info : [readwritesplit] Selected RUNNING SLAVE in [10.40.12.102]:3366
2019-01-11 13:59:43 info : [readwritesplit] Selected RUNNING SLAVE in [10.40.12.103]:3366
2019-01-11 13:59:43 info : Started Read-Write-Service client session [50] for 'lbadmin' from ::ffff:10.40.20.203
2019-01-11 13:59:43 info : (50) [readwritesplit] > Autocommit: [enabled], trx is [not open], cmd: (0x03) COM_QUERY, plen: 37, type: QUERY_TYPE_READ|QUERY_TYPE_SYSVAR_READ, stmt: select @@version_comment limit 1
2019-01-11 13:59:43 info : (50) [readwritesplit] Route query to master [10.40.12.101]:3366 <
2019-01-11 13:59:43 info : (50) [readwritesplit] Reply complete, last reply from server1
2019-01-11 13:59:43 info : (50) [readwritesplit] > Autocommit: [enabled], trx is [not open], cmd: (0x03) COM_QUERY, plen: 49, type: QUERY_TYPE_READ|QUERY_TYPE_SYSVAR_READ, stmt: select @@hostname – maxscale route to slave, Hint: HINT_ROUTE_TO_SLAVE
2019-01-11 13:59:43 info : (50) [readwritesplit] Route query to master [10.40.12.101]:3366 <
2019-01-11 13:59:43 info : (50) [readwritesplit] Reply complete, last reply from server1
2019-01-11 13:59:43 info : (50) [readwritesplit] > Autocommit: [enabled], trx is [not open], cmd: (0x01) COM_QUIT, plen: 5, type: QUERY_TYPE_SESSION_WRITE, stmt:
2019-01-11 13:59:43 info : (50) [readwritesplit] Session write, routing to all servers.
2019-01-11 13:59:43 info : (50) [readwritesplit] Route query to master [10.40.12.101]:3366
2019-01-11 13:59:43 info : (50) [readwritesplit] Route query to slave [10.40.12.102]:3366
2019-01-11 13:59:43 info : (50) [readwritesplit] Route query to slave [10.40.12.103]:3366
2019-01-11 13:59:43 info : Stopped Read-Write-Service client session [50]

=============debug log end

there's some issue in my configure? or bugs ?
thanks for any reply!