We have more detail from the factory, explaining to us that it is safer to inspect the inside of the button of all units instead of just the ones with damping feeling.  However, the fix seems to be faster with the help of some heat and cleaning solution.  

Also, they have fixed some units, and the fixed Aumeos are currently in a hot-cold chamber to simulate the "aging" process. Hope there will be no more chemical reaction to cause sticky button.  Will keep everyone updated on the result.