I am having a similar issue, but it appears to be more than just NAT loopback. All of my port forwards seem to clear out (12-72 hours). I go back in to the settings (where everything still appears) and click apply and then everything works again.
I had a problem when I first got the router, but a reboot restored loopback, and I haven't had any problems since then. I've run a couple different versions of -Merlin, now on .47
I am seeing this exact same issue on all of the latest builds (including RMerlin's builds). It appears to be more than just loopback as I cannot reach my webserver from the outside when this happens. I will try other ports the next time this happens. I also have a 2am gremlin "rc_service: rc 3603:notify_rc restart_wrs" in my logs.
It would be nice if this got fixed as it is my only real gripe about this router so far. Fingers crossed!