Hey @Hao Wu — welcome to Figma Forum, and thanks for flagging this.
I can't discuss billing specifics here, but I did check in with our Product Support team and they're actively reviewing your report. Your case number is: 1946943, and they'll follow up on your ticket once they have more details and next steps.
In the meantime, this resource may help explain the charge. If any seats were added before your plan downgraded, those would be prorated and reflected on your last invoice: Upgrade or downgrade your plan → Downgrade or cancel your plan. That said, Product Support will be able to confirm the specifics.