Multicast storm created by firewall

[Andreas_]

From time to time, we're suffering from some strange issue:
Triggered by a workstation on LAN1 sending a ws-discovery multicast on port 3702 (or some other service, just as example), some thousand duplicated packets can be seen on LAN2 (with LAN1-address as sender and mcast as destination), with the source MAC address of the backup firewall of a CARP pair.

Or in other words:
The carp backup firewall, which should be listening passively, creates IP Multicast packets with its own LAN2 MAC source address, LAN1 IP source Address of a client, with a rate of about 5000/s and will not stop until the firewall is kicked with pfctl -d;pfctl -e

Hotfix is to drop UDP traffic to specific ports (such as 3702) on the LAN1 network, but a firewall shouldn't create such packets on its own, right? It's 19.1 (had this already with 18.1/18.7), no specific Multicast/IGMP settings or modules.