Interestingly it doesn't seem to work in iOS 8.4 either, but as Jeremy says above, it appears as though in the latest iOS 9 beta it works fine.
I can understand where it might get confused. The search box, in theory, should focus when the search icon is pressed (and it does on Chrome and Firefox). It would suggest that the automatic focus is possibly happening without bringing up the keyboard, so it then requires a second tap.
We'll see how it works in the next public beta.