They already explained that it's like an infection that lasts for like 1 month and then goes away during the chapter where that girl was agitated from withdrawal symptoms.So mc have eaten Mayuri's donut 6 years ago. And if we throw back to first chapters, it's already middle school when he start questioning about that. So maybe after that mc can't see it again and when in the middle school something happen again and starting to see the donuts again?
Yes I know. The point that I want to pointed out is this.They already explained that it's like an infection that lasts for like 1 month and then goes away during the chapter where that girl was agitated from withdrawal symptoms.