What seems to be happening is the light is hitting the bottom of the glass, going through it, and then reflecting upwards hitting the top of the bottle, creating a weird reflection. The reflection changes based on the angle you look at the object.
I'm pretty sure that this is working how it's supposed to. It just looks weird to the eye. There are a few things you could check/do to fix it though.
- Check to see if the liquid, is touching the glass. This is known to cause artifacts.
- You could change the camera position or lighting to make it less
- You could also adjust light bounces in the light paths drop down in
the render panel. (this would decrease the realism, but you could lower the bounces so that the light doesn't reflect enough times to cause that reflection)
I think you might benefit from this tutorial. I linked it at the point where he started doing the liquid.