I would suggest then that the young leaf already had damage or was infected. From what I understand of how it works, hydrogen peroxide is broken down into water and oxygen when it comes into contact with an enzyme called catalase which is present inside cells including bacteria and fungus and is in blood fluid. If you hear fizzing that is the catalase causing the reaction and oxygen being released. If cells are damaged then catalase will be present. Peroxide will damage bacterial and fungal cell walls which is what kills them. Hydrogen peroxide is inert otherwise. If you get it on your skin you will not see any reaction unless there is a tiny cut and blood is present, then you will feel a sting and possibly see bubbles.

If you see a scar on a leaf later on after applying peroxide then you may have stopped an infection that was present at the time.