I have two RT-BE86U's running Asuswrt-Merlin 3006.102.4. Last night one of them decided to spontaneously factory reset. I was greeted with the setup wizard when logging in..
There seems to be at least one other case with same firmware version:
www.snbforums.com
I have four Asus routers, only one was affected. All my routers are behind UPS power so there haven't been any power outages.
The BE routers are 10 months old, most of that time running Merlin. No big issues before.
There seems to be at least one other case with same firmware version:

RT-BE88U factory reset itself while I was away – any ideas
Hi all, Came back from holiday to find my RT-BE88U showing a solid red WAN LED. I hardwired into it and got taken straight to the initial setup wizard, like it was brand new. All my settings were wiped – SSIDs, DHCP, port forwards, the lot. It seems to have factory reset itself. I'm running...

I have four Asus routers, only one was affected. All my routers are behind UPS power so there haven't been any power outages.
The BE routers are 10 months old, most of that time running Merlin. No big issues before.