DPC3941B partially answering static IP HTTP GET requests

Comcast Business internet with 5 static IP addresses, and Comcast/Cisco DPC3941B Router/Gateway.

As I moved a server from the default static IP and port forwarding to one of the 5 static IP addresses, it was a nightmare. The help documentation doesn't match this Router/Gateway, leading one to stumble/guess their way through setting it up.

So, for reference:

Comcast Router (Model DPC3941B):
Advanced -> Port Management -> Check "Disable all rules and allow all inbound traffic through"
Gateway -> Firewall -> IPv4 -> Check "Disable Firewall for True Static IP Subnet Only"

Theoretically "Disable Gateway Smart Packet Detection" is checked, as the device shows it checked but disables the checkbox.

Of note, requests from the LAN inside the Gateway/Router worked just fine to the server without the firewall options above. Only from the outside do those apply.

Now, for the problem and strange behavior...

Now, if this server is connected, all is working, web and ping requests are answered. If I unplug this server from the Router/Gateway, pings fail, as expected, but web requests are PARTIALLY answered, NOT EXPECTED!

Watching a packet trace from the outside, the TCP connection is established, then the HTTP GET is sent and the Comcast Router/Gateway ack's the GET request. Remember, the server is not physically connected. After the ack, eventually the keep alive packets start, while the browser continues to wait and wait. Eventually the browser gives up.

This gives the impression that it's a webserver problem, or maybe a server firewall problem, which it's neither. The Router/Gateway is answering TCP connections and ack'ing HTTP requests for a server that's not even there (either physically, or completely blocked by the Router/Gateway's firewall).

Why, and how do I disable this behavior?