ER-X 1.8.5 IGMP proxy and per-port VLANs

ER-X, firmware 1.8.5. I have configured the switch for per-port vlan as follows:

interfaces {
    switch switch0 {
        mtu 1500
        switch-port {
            interface eth1 {
                vlan {
                    pvid 100
                    vid 20
                }
            }
            interface eth2 {
                vlan {
                    pvid 100
                }
            }
            interface eth3 {
                vlan {
                    pvid 100
                }
            }
            interface eth4 {
                vlan {
                    pvid 100
                }
            }
            vlan-aware enable
        }
        vif 20 {
            description "IPTV VLAN"
            mtu 1500
        }
        vif 100 {
            description "Intranet VLAN"
            ... LAN config here ...
            mtu 1500
        }
    }
}
protocols {
    igmp-proxy {
        interface eth0.4 {
            alt-subnet x.x.x.x/24
            role upstream
            threshold 1
        }
        interface switch0.20 {
            alt-subnet x.x.x.x/24
            role downstream
            threshold 1
        }
    }
}

The idea is to forward the downstream multicast traffic only to port eth1 of the switch (to tagged VLAN 20) which will be taken care of by another managed switch.

This doesn't seem to do what I want - the traffic still floods all interfaces and router basically hangs when IPTV stream hits it.

I thought the per-port VLAN config should block all non-tagged traffic. Any suggestions? What I need is something like IGMP snooping on the hardware level.