While using the standalone version of AdGuard Home with vanilla OpenWrt 23.05.4, I never saw localhost (127.0.0.1) DNS requests, now using the GL version (integrated) of AdGuard Home, I see a lot of them (#4 in the list, from the top), is this just a matter of the way how AdGuard Home is configured by GL? Or a misconfiguration? Thank you
This is usual because the router will rewrite all incoming DNS requests to use AGH.
It's not really a matter of configuration - more like how AGH works within the GL firmware.
It's a frequent question here...
They asked me to reset the firmware but I didn't do it, because I don't like to reconfigure everything from the beginning.
But, why is localhost (127.0.0.1) as #2 in the Top Clients list? That's what I don't understand
Depends on what your top is 
Did you switch the mode from standalone to GL? This might not delete the stats.
I flash 4.6.3-op24, therefore full reset, everything from scratch
So what is on 1st place?
Another random client device