That's nothing new; every iPhone I have had with every iOS version that supports WiFi syncing has done this. I suspect its because WiFi on iOS devices is turned off to save battery when they phone goes to sleep, unless it is connected to power.
I too have noticed in the past that rarely my iPhone would not show up in the iTunes device list. I'm pretty sure that wasn't when it was asleep, though.
WIFI sync has ALWAYS only worked when an ios device is PLUGGED in to an ac receptacle!! It is a requirement for it to work!!
That's false. You may be thinking of the fact that plugging in a device automatically launches a WiFi sync. But you could always initiate a WiFi sync from the device or iTunes even when it was not plugged in.
I stand corrected!! As many ios devices as I have, I have never tried to sync wifi without plugging the device in. Learn something new every day!! Just did it with my 4s which is my worst case scenario, so I'm still fixed with the new itunes and 8.1!!