There seems to be an issue of coincidental geometry. Avoid having the liquid and glass sharing faces that sit right on top of each other. Scale the liquid so that it pokes through the inside of the glass just a tiny bit. Also by doing that, you'll correct the air gap in the current render. Basically, the liquid is supposed to look like its touching the "outer layer" of the glass as seen in my test render on the left and the reference image on the right.