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Thanks @DdmrDdmr @fillippone, I updated the merit activity and monthly post for Indonesian local board. It seem that an incident like this has happened before when I haven't updated monthly post activities and merit for local board or to be precise in July and August 2020. Of course, because the activity chart that I made only specializes on Indonesian local board, then -91 will make one upside down candle on the monthly chart. Again, thanks for the explanation.
As I am doing this kind of updates for the Italian board, I regularly notice this kind of “cleaning” activity taking place from time to time across all boards.
My suggestion is to ask some “colour” on this to your local moderator as he could give you some more insight.
Ah! This is going to help our board overcoming yours! (Long shot extrapolation needed!)