This is another issue that we have been seeing for many versions / combinations of versions of firmware. Currently SM's are at 3.5 and AP's are 3.4.1 this problem has existed all the way to at least 2.6.something and possibly before. We only have a few customers that actually require port fowarding (DMZ works for all our console gamers and I don't know if DMZ stops working as they probably just reboot stuff when it breaks) and it seems to affect all of them.
The port forwarding works perfectly for weeks at a time then suddenly it stops forwarding any ports , none of them appear to be gettign forwarded and so far rebooting the radio is the only thing that fixes it. We have tried various settings regarding DMZ, UPNP , and NAT PMP (PCP) on/off.