Whatever you choose in advanced wireless settings wifi always goes to sleep, that was also the case with previous builds and i think that someone has to report it at MIUI. This is really annoying bug because after 5~10 minutes with screen turned off, wifi will go to sleep and as a result you are not able to receive emai etc notifications and mobile data is being enabled (if you have it enabled) and this eats MBytes from my limited data plan even if im near a WiFI access point.