It's a new feature introduced with the 311 bios update. This is because of when it's plugged in with AC, the laptop not go to sleep mode, and still running with screen off, and it was causing heating issue, so they solver by allowing the fan on. If you want the laptop to go sleep mode, unplug the ac and it will sleep well. Btw, I found that if it in balance mode, go to sleep too.
I hope I'll help you a little bit.
Yes it helped!
Unplugging AC switched it to "Balanced" mode. With plugged in AC laptop switchs to "Performance" mode. Is there any options\parameters I can change to allow it to go to sleep mode with pugged in AC? I found it weird.
The processor cores are heated to a temperature of 50 degrees Celsius in idle mode. And cooling system works loud. I bought myself an extra fan stand. It helped drop a couple of degrees.
You can change the modes in the 'Armoury Creator' app, just press the rog button, or with the 'fan' logo button, and it has to switch automatically. If not, probably you have to install all of this software. I recomend use the app, 'MyAsus' from Microsoft Store and install the necessary software about Asus.