My only experience with plucking and my grey was related to stress. When I got her...she is a rehome....Shadow had several "chewed" spots and one bald spot on her upper leg, however, after changing her food to a natural dye free variety and time to settle in, her feathers all grew back. Even the bald spot. I am sure here transition from her first home through another home to mine was very stressful. I saw some of this same type of behavior this spring when our life was VERY crazy and everyone was somewhat neglected. We were all very stressed out and Shadow shared in those feelings and again began some chewing and plucking. Again, the behavior stopped when the stress was relieved.
I do hope you can find some peace with your situation. As others have mentioned, ignoring the behavior will be much more helpful than focusing on it. Hopefully you can find something that helps. There should be comfort in knowing that the vet did not find anything medically wrong. : )