it's not an anti-siphon valve.
It's called an Air Admittance valve. It lets air in but nothing out.... if it's working right. If it moves it might be loose.....
It should be threaded into the plumbing pipe tight enough to seal. It' wouldn't need to be super tight, but tight enough to seal, and moving with your fingers isn't sealed.
but
it has a cap over the diaphragm that just fits over the valve and that could be a little bit wiggly.
Inside it's just a simple diaphragm. Like a flap that closes. Lets air in but nothing out. I found that if you unscrew the whole, flip it upside down so that the stinky side is up forming a cup, you should be able to fill it full of water and it should hold. If water is getting out this way, then stinky air certainly can get out too.
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