For some time, I've been using DD-WRT (DD-WRT v24-sp2 (06/14/11) std - build 17201) on a Netgear WNDR3700 v2 router. Yesterday, I found that the 2.4 GHz network had become open and that the Shared Key field of Physical Interface ath0 was blank. I've put the shared key back in and find that it usually disappears when I hit Apply but doesn't disappear if I click Save. I've tried rebooting the router, choosing different combinations of Security Modes and WPA Algorithms with no luck. The shared key stays with the 5 GHz radio.
Here's what the Wireless Security page looks like:
The build name and version doesn't match what's on the dd-wrt site but the build date does.