An off-the-wall suggested cause-and-effect experiment, with a little caution required.
On the assumption that you are not accessing the cameras from an outside network (eg internet, so traffic would have to pass through the Fritzbox as default gateway) and knowing you have static IP addresses on the cameras, so no dependence on DHCP, and no dependence on UPnP:
In the camera config screens, security, it's possible to create an IP address blocklist. And actually a whitelist, though that's a bit more risky.
I believe the Fritzbox IP address could be added to that list without any major adverse effects. If the camera has an internet time server configured, that will be blocked, so time may drift a little.
But it might be able to test your theory that the Fritzbox is somehow implicated.
I'm not so sure about what would happen if multicasts were blocked, to kill the multicast DNS on 224.0.0.251, or even if the camera config would allow that. ARP is a broadcast, so that should still work.
Just a thought ...