looks like the cavity of that valley isn't the same to the rest of the others for that cog. I bet a slightly more worn chain on that particular cog would not experience the pop when it rotates to that specific area.... although, it might not play well with the majority of the remaining valleys.
Wonder if the OEM will issue a TSB to ever so slightly file that one valley or waste a whole cassette by replacing it with another defective cassette.....
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