7.5.2 no longer routes internally when using external IP

I've been using my AirPort Extreme's external IP address (actually a name using DynDNS) for all my URLs when I want to access my local web server. That way I only need to have one URL regardless of whether I'm at home or away. Worked great.

Since updating the firmware, I can no longer have an internal device access the local web server via the external IP address. The AirPort is not routing the request to the internal IP:port when the request goes to the external IP.

I've confirmed that the port mapping in the AE are still present. Using Terminal, I can ping the external address successfully, but when I Telnet into port 80 to the external address — nothing.

So I rolled back the firmware to 7.4.2 and it's working once again.

I'll be opening a bug report to Apple unless anyone knows a quick fix.