ESP8266WifiServer timeout when requesting through external IP

I have a basic `ESP8266WebServer` that sends roughly 1000 characters worth of response via `send()`. Everything works fine when the traffic is over the local IPv4 address _192.168.xx.xx_.

However, I have set up port forwarding over my external IP to be directed to the NodeMCU device, and when the request is made over the external IP _87.xx.xx.xx_ via a device connected to the local network, the server times out during `send()` operation. Upon closer inspection, it seems that `ClientContext::availableForWrite()` / `tcp_sndbuf` returns `0` in this case. Increasing the timeout to `10s` didn't help. Seems the buffer is never available.

As an additional step, I set up a VPN on the request device to make the request device's IP as _105.xx.xx.xx_ and made the same request. This time `ClientContext::availableForWrite()` / `tcp_sndbuf` returned a non-zero value and the request was successful.

Any ideas why the `tcp_sndbuf` would not work when a request is made by a device connected on the same network but via external IP that is port forwarded to my NodeMCU?