As reported last night.
If I sysupgrade from 21.02.0-rc2 to a later release such as 21.02.1 and 'keep settings', I have observed LuCI survives subsequent reboots (until I reset the device to default OpenWrt settings)
I also observed when I back up the settings while LuCI survives reboots, and then restore the settings, I get 'LuCI Configuration interface' followed by "Bad gateway". WinSCP then refuses to connect until I reset the router to restore openwrt defaults.
I couldn't find any differences between the files in /etc/config. Only the ULA prefix in /etc/config/network differed.
21.02 seems to be snafu on MyNet N750....
I've put 19.07.8 ath79 image on the N750 and it doesn't exhibit the LuCI or backup/restore issues so far. I know 19.07.7 ar71xx image works reliably.