Can't select bridge member interface in NAT rule

Is PortC and D your LAN? Because you should not do SNAT based on Incoming interface. The better approach is outbound interface. 

That's not always the case.
Besides, this issue is only with the bridge interface.

There seems to be a technical limitation, which i never discovered in the past months. Basically because i never did a SNAT based on the filter of the interface. I cannot come up with a scenario, why you would do this in a bridge scenario. 

So what is the use case of doing this? Why should a SNAT be applied on only one port and not the other port, if traffic is generated by both ports? 

Why does selecting PortA or PortB work fine, but selecting br0, PortC or PortD causes problems?
What is the reason for such behavior?

Properly a technical limitation. As nobody has discovered this limitation yet, it is likely to be some rare condition. Likely it is because of the way a bridge works. The interfaces will always have the same subnet range. Therefore to separate PortC and D does not make much room of a valid use case. 

I can still not think of any kind of use case of doing this. Why should i do this? 

I think the behavior is inconsistent.

"Create a source NAT rule" in the Administrator help also sets the inbound interface.

docs.sophos.com/.../index.html

For example,
1.
No NAT required between PortA and PortB.
NAT is required for PortC or PortD.
It is necessary to select "Any" for the inbound interface in the NAT rule, but that also applies between PortA and PortB.

2.
Another router is connected to PortC, and NAT is required only for access from the network through that router.
I want to select only the required PortC as the inbound interface in the NAT rule.

I want to limit the conditions to the minimum necessary.

The picture is actually very misleading. Can you show us your Network interfaces? 
And what do you want to do with the SNAT (assuming MASQ?)

I don't want you to tell me how to do something.
I just want Sophos to fix the NAT behavior so that the inbound interface settings work.

Then please create a support case so this can be looked and prioritized by support. 

I can't create a support case.
All I can do about bugs is write here.