I don’t get a buzz when removing the mouthpiece at all on standard 2nd line G for example, but do get a buzz when removing during a lip bend (say down a tone or more). I don’t know if that means anything. Lip bends really sure up my aperture corners and I love em.
Single and double harmonics sound good. Fast a clean. As long as you are not pushing from the abdomen you are winning on those ones. I would move on if I was you. The upcoming Octave harmonics are challenging around high A, where you start to need to ‘support’ from the abdomen. Getting this balance right is always something I work on daily.