Apple Airport Express not issuing DHCP

I came across an issue with an Apple Airport Express the other day, where it wasn't forwarding DHCP requests to the network's DHCP server.

System Overview

Router

Hard-wired devices directly connected (e.g. printers)
Airport Express directly connected
Configured in Bridged mode, so that it was only acting as an access point, and was NOT issuing DHCP itself (nor was it providing an additional layer of NAT)
Wireless clients connect via this access point, but use the Router's DHCP server

Normally this has worked very well for me.

Fault symptoms

Recently (possibly since upgrading to firmware version 7.7.3), after a reboot, the Wifi clients were unable to attain an IP address. They connected to Wifi without any problem, but are not issued a DHCP address

All other devices (hard-wired receive DHCP addresses OK
All other network operations (print / file sharing / internet etc) for wired devices work OK
The Time Capsule service works fine.

After escalating this to Apple (with the vendor suggesting a hardware fault), we tried purchasing a new airport express. This did not fix the issue.

We also tried several reboots, factory reset, etc. We also followed all apple instructions (read the manual, etc).

Resolution

It seems that now (perhaps with the new firmware), to use the Airport Express as an access point in bridged mode, you MUST

disconnect the WAN interface of the Airport Express before powering it on.
Power on the Airport Express
Wait about 30 seconds
Connect the cable from your router to the Airport Express WAN port.

It seems that when a cable is connected to the WAN port, it activates a hidden parameter which prevents the forwarding of DHCP requests through to the WAN (i.e. in this case, the router).

If you encounter a power failure or reboot, you must follow this same process every time.

So far Apple haven't come back with any acknowledgement of this issue, so maybe this is related to a new feature.