Broadcast flood generated by firewall

[Andreas_]

There are some smartphones that will connect via wireless to one LAN or another, depending on app needs. Apparently, IOS phones may remember the old IP address, and sending out UDP broadcasts for quite some stuff (SMB, dropbox, spotify) using the old IP address (network A) on a LAN that has another network B.
Even if the iPhone is disconnected, about 4000 packets/s are still broadcasted, originating from the firewall's B network, but broadcasting A-sourced packets.
I have invented block rules
- for specific UDP ports
- for 255.255.255.255 destination
- for any packets that don't originate from that interface's network

Still, these broadcast storms from the firewall persist.
To stop the storm, I need to issue pfctl -d ; pfctl -e

I'm running out of ideas.

card/pfsync pair of opnsense, sometimes the master is the source of the broadcasts, sometimes the backup.

Anybody a clue for me?
Regards
Andreas