It's 12.5 gallons. My wife has a RX500h. Happens on every fill. Same with the Highlander hybrid. All over their forums. Techs claim fule gets trapped in the baffles.
If it's the same setup as some of their other hybrids, fuel capacity stinks. Can't use all the fuel in the tank. 17 gal tank and when the the low fuel light comes on you can only put 12.5 gallons in it.