I had a really strange problem with an Asus router — the port forwarding disappeared. And while I could use the UI and put everything back in, it didn’t stick around. Turns out the NVRAM was full — there wasn’t anywhere to put the port forwarding rules (vts_rulelist). Fortunately, there were a few old DHCP reservations I was able to delete and free up some space. For future reference, the following command reports what is using the NVRAM.

nvram show | awk '{print length(), $0 | "sort -n -r"}' | cut -d"=" -f 1