OK. Just move native-vlan-id under parent interface config...
set interfaces ae31 flexible-vlan-tagging set interfaces ae31 mtu 9216 set interfaces ae31 encapsulation extended-vlan-bridge set interfaces ae31 native-vlan-id 3174 set interfaces ae31 unit 3174 vlan-id-list 21-22 set interfaces ae31 unit 3174 input-vlan-map push set interfaces ae31 unit 3174 output-vlan-map pop