Do you have the IP firewall enabled for the bridge? If not, then all traffic on the same subnet will simply pass through as layer 2 traffic and bypass the firewall completely if I’m not mistaken.
I = incoming traffic from elsewhere
J = local process on router (incoming traffic)
K = local process on router (outgoing traffic)
L = outgoing traffic to elsewhere
In your case it should be enough to change chain from prerouting to output.