After the Candy Carnival Challenge is completed, the candy doesn't seem to be accumulated? Don't be afraid, the points are actually accumulated!
When you complete the challenge, a small pop-up will appear on the right side of the screen, showing "Candy +10" or "Candy +20". When you go back to the challenge, your candy has been accumulated to the upper right. Because the mission has been completed, the progress will be reset to 0/1 or 0/100 immediately.