I came to this thread because of the same problem. Sounds like crushing salt under the button.
While reading this thread I thought of blowing the button.
So I pressed down and blew hard all sides.
Problem gone immediately. I think it's just a very small dirt. I can't even see it.
It is from moisture (ex sweat) that gets trapped in there. EVERY TIME that I blow in there as well, the problem is gone.
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