UDP stream not forwarded to network

I have an KPN combined fibre internet/TV connection. It uses routed IPTV to provide tv service. As such I have IGMP snooping active on my two switches and configured igmpproxy on my firewall Ubuntu 18.04 box.

This is my config

quickleave

phyint eth0 disabled
phyint eth0.6 disabled
phyint ppp0 disabled

phyint enp5s0 downstream ratelimit 0 threshold 1

phyint vlan4 upstream  ratelimit 0  threshold 1
        altnet 0.0.0.0/0

A TV stream starts as unicast, but after a second or so, switches to multicast. I can see an UDP stream of about 8.000kb/s run on vlan4, but it never gets to enp5s0, hence the stream freezes. It's almost as if igmpproxy is ignored. However, as soon as I kill the igmpproxy, the stream also stops.

I would expect it to be forwarded from the vlan4 interface to the enp5s0 lan interface. But this does not happen. The issues is somewhere between the routing and igmpproxy. The routing is, for this test setup, permissive, as in the policy on all interface is ACCEPT.

iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  anywhere             213.75.112.0/21
MASQUERADE  all  --  anywhere             anywhere

iptables -L -n
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

This an overview of the network itself

This is an overview of the current routing when igmpproxy is running;

Current routing table (Insert Route):
-----------------------------------------------------
#0: Dst: 225.0.71.1, Age:2, St: I, OutVifs: 0x00000001
#1: Dst: 224.3.2.6, Age:2, St: I, OutVifs: 0x00000001
#2: Dst: 224.0.252.127, Age:2, St: I, OutVifs: 0x00000001
#3: Dst: 239.255.255.246, Age:2, St: I, OutVifs: 0x00000001
#4: Dst: 224.0.0.251, Age:2, St: I, OutVifs: 0x00000001
-----------------------------------------------------

I can see similar tables being active on the DSG-1100 units. I am looking at this issues from mulitple angles, but so far have not find a solution or cause.