The poster i was responding to equated subnetting to vlans. I might have misunderstood what they meant though. It sounded like they wanted to use the same subnet per vlan, which wont work if you want them routed in the same gateway.
Reading it again they make it sound like you can’t subnet all of these networks on a switch without vlan, which you definitely can. I could for example connect 4 different devices on the subnet 192 168.10.x/24 and have them reach each other. I could also connect 4 more devices in the same switch but on a different network 192.168.20.x/24 and it would work.
It has to do with link priority on the server. You’d imagine that a server that receives a packet that has a return address on the same subnet as it self logically would use that interface instead.
A similar thing happens in switches. For example if you have two vlans on a switch and both vlans have an ip assigned, connect a computer to one of the vlans. You will only be able to reach the switch on the non-routed connection. Even if you also are allowed to reach the second vlan through a router/Firewall.
My guess is that the server receives the packet from the client with src .11.101 dst .10.102 and tries to respond over the interface that has .11.102 assigned. The client expects a response from src .10.102 and drops the packet. But I would turn on a packet sniffer in the gateway to see if the returning traffic even passes the Firewall in scenario 1.
By making a bridge in the opensense interfaces you have created a layer2 network. This means that all the devices connected on that network are broadcasting their Mac addresses and are added to the ARP table on the opensense. Since they all are on the same physical network and the same subnet, none of the traffic will ever hit the layer 3 rules on your opensense.
If you want opensense to handle the rules of the traffic you will need to put the devices on different subnets and separate clans. Create a gateway address for every vlan on the opensense and point your devices to the opensense as their gateway.
Sorry about my confused rambling 😅 Yes, the example was to demonstrate the difference between subnetting and vlan. Albeit simplified. What you said is right.