AsNoriuanswered grow question 4 months ago Looks like bud rot.
You dont do any flushes in soil. Its for hydro or coco, even there its needed if you overloaded plant with food.
Years ago, when i believed in it , I was doing heavy flushes in soil. Like 3 waves of 20-30 liters of phed water through 30-40 liters of soil. It never caused any bud rot.
It happens, thats worst thing in growing weed. Read about it and take heavy/quick actions ASAP ! Take loses if needed, dont be gready.