mDNS Repeater only works when I look at it

Yes this is going to sound odd, but my mDNS repeater across 2 VLANed subnets works great, but only while I am in the Webfig or Winbox looking at it.

Actions:

• Added logging input FW rule that accepts dst-port 5353 UDP packets, this logs remotely so I can see the incoming mDNS packets. This rule is right at the top so it should always get hit and not use any established connection alternates
• Added the 2 VLANs interfaces to the mDNS repeater, this works, and I can use mDNS over the 2 subnets
• As soon as I close out of the Webfig I stop seeing the input firewall rule being hit, the log gets no entires from the firewall rule, however I still see other log entries, mDNS devices slowly stop seeing each other

Network:

• Laptop connected via wifi to cAP, which is connected via ethernet to the main RB4011 (with mDNS repeater)
• mDNS devices connected via a variety of wired (directly to RB4011) and wireless via the cAP on both VLANs
• RB4011 running 7.21.5

Other oddness:

• If i run /ip dns set mdns-repeat-ifaces="" then /ip dns set mdns-repeat-ifaces="VLAN002,VLAN003" it seems to kick start it for a minute or so with packets being seen in the log

• If I change any firewall rule, even just a comment it seems to stop the mDNS packets being seen

• By far the most reliable way to get it to work again is to just click apply on the DNS configuration page in Webfig

• Using some packet sniffing I found that incoming IGMP packets seemed to stop it working, so I have dropped IGMP input above the mDNS accept input rule, also disabled anything IGMP on the network, turned off snooping etc.

Anyone seen anything like this? Thanks.