So, I have experimented, and I have confusing results...
Very interesting, indeed, right before returning my original R7800 with failed NVRAM in the RMA swap, I decided to try it out one last time. And lo and behold, the darned thing seemed to remember its settings--even after completely shutting it down, unplugging it, etc.!!! So after sitting unplugged and unused for several days, it seemed to recover...
I had flashed it down to the latest official firmware at that point (i.e. v1.0.2.68) for return to Netgear. But with that same .68 firmware in the previous days while it was still in continuous operation, it had not preserved the settings. So it does not seem to be related to firmware version.
I do not understand why it started functioning again. The only logical explanation I can think of is that the issue might be heat-related... When it originally lost the settings, it had been on and running continuously. Over the next few days, I kept it plugged in and running as I worked out the problem, and then waited for the RMA swap to arrive. So it was always on, and thus, always warm/hot, during that time. And if I did a power cycle, it would lose its settings. Only after letting it sit around unplugged and unused for several days did it seem to recover. For that reason, I speculate that the issue may be heat-related.
The saga continues because with the brand new RMA swap R7800 I received, I had it up and running with Voxel's latest v1.0.2.75.2SF firmware, and it was working great. Even on full power cycles with full unplugging, it came right back up as expected with a preserved configuration. But then after just two days (on Thursday afternoon), it failed in exactly the same way as my original R7800. I.e. out-of-the-blue*, it was reset to factory settings, and then would not remember the configuration on a power cycle, nor was it able to ingest any of the saved configuration files I had made over the last two days.
* I wrote "out-of-the-blue" in the last paragraph, but it was not completely random. We suffered an electrical brownout at that moment, and I heard the UPS feeding the router make a single beep before it recovered. The voltage sagged enough that the router lost power, and that is what seemed to have caused the problem. This indicates that the UPS battery was probably failing so I swapped in a whole new UPS unit. I suspect this is also what happened about two weeks ago when I was at work, and got the emergency message from home that "the Internet was not working."
Anyway, on Thursday afternoon after the failure with the new RMA swap R7800, I had no option but to pull it and substitute in a backup router to maintain peace with the family. And I left the R7800 unplugged sitting in the corner, and feeling totally dejected that it has lasted only two days. Then, yesterday evening, after it sat unplugged for two days, I figured I would try it again...
I did not boot it directly to check out how things were. Instead, I had a hankering to try out OpenWRT on it so I immediately flashed that, and did a factory reset after that. Then I booted it up, started playing with it, did full power cycles including full unplugs, and was stunned to see that it was preserving settings at that point!!! Even though it had not been doing it just two days earlier!!! What was the difference? Nominally nothing except that it had sat unplugged in the corner for two days. So this somewhat supports the idea of a heat-related problem.
If it is a heat-related problem, it should affect any firmware--official, Voxel's, OpenWRT, or whatever, right? So I am doing a test now, and have left it with OpenWRT. If it fails with OpenWRT, well, for sure, it is a hardware problem.
The other difference, as mentioned above, is that I have swapped in a new UPS. But gosh darn it, it seems strange to me that the device would be so susceptible to power fluctuations. And after the brownout, it clearly could not maintain its settings nor read in old config files. Anyway, I will keep testing it with OpenWRT to see what happens.
One slick thing about OpenWRT is that all the config files are simple text files. So I was able to grab the /etc/config/dhcp file from another OpenWRT install, in which I had previously spent the time to change all the machine names to something recognizable. I just scp'ed it over to R7800 which I had just flashed with OpenWRT, rebooted, and had all my machine names set up!!! This saved me a huge amount of time!!! It would be cool to be able to do this with Voxel's firmware, too, because trying to restore settings via the web GUI does not seem to work**.
** Actually, it just occurred to me that if the issue is heat-related, maybe after waiting till the R7800 cools down and rests, the restoration of a configuration via the web GUI would work, too?