Sillcock leaks water when hose is above faucet

I have a brand new Quartermaster Sillcock (anti-siphon, quarter turn). I got it from the Home Depot (mistake #1 I am sure).

If I have a hose attached it with the line pressureized but not flowing and I bring the end of the hose above the sillcock, water starts to come out of my sillcock on the outside of the house. If I lower it the water stops coming out.

Is this part of the antisiphon? Or is something else wrong?

I am about to hardibacker behind the tub, which will forever seal my access to replacing this sillcock...

As I understand it, if the pressure outside ever gets higher than the inside (supply), the the valve closes. This means that the water in the hose leaks out until the pressure is lower or the same as the supply and starts flowing again. It might just be a normal situation. The goal is to prevent water from "outside" ever getting back into the supply and contaminating it. When the hose is flowing, it is always at or lower pressure than the supply. Raising the end will raise the pressure, the higher you go, the greater the pressure. Doesn't take much, or the valve wouldn't be any good. My unprofessional opinion.