Is your nlbwmon databases stored on /tmp?
I'm using a snapshot for the Raspberry Pi 4 and my /var is /tmp so my databases don't get stored.
Is it the same for other routers?
cd /var/lib/nlbwmon/
@OpenWrt:/tmp/lib/nlbwmon#
root@OpenWrt:/tmp/lib/nlbwmon# ls -l /
drwxr-xr-x 2 root root 4096 Feb 13 15:35 bin
drwxr-xr-x 3 root root 16384 Dec 31 1969 boot
drwxr-xr-x 7 root root 2700 Mar 8 19:53 dev
drwxr-xr-x 19 root root 4096 Feb 13 15:35 etc
drwxr-xr-x 11 root root 4096 Feb 13 15:35 lib
lrwxrwxrwx 1 root root 3 Feb 13 15:35 lib64 -> lib
drwx------ 2 root root 4096 Dec 31 1969 lost+found
drwxr-xr-x 2 root root 4096 Feb 13 15:35 mnt
drwxr-xr-x 2 root root 4096 Feb 13 15:35 overlay
dr-xr-xr-x 92 root root 0 Dec 31 1969 proc
drwxr-xr-x 2 root root 4096 Feb 13 15:35 rom
drwxr-xr-x 3 root root 4096 Feb 25 23:52 root
drwxr-xr-x 2 root root 4096 Feb 24 11:08 sbin
dr-xr-xr-x 12 root root 0 Dec 31 1969 sys
drwxrwxrwt 17 root root 480 Mar 9 11:46 tmp
drwxr-xr-x 8 root root 4096 Mar 2 16:06 usr
lrwxrwxrwx 1 root root 3 Feb 13 15:35 var -> tmp
drwxr-xr-x 4 root root 4096 Feb 22 13:33 www