I set my nightly rate for September at $175 and set a monthly discount at 35%. I got a reservation inquiry for Sept. 12 - 30, and the system applied the monthly discount, setting the nightly rate at $114, despite the fact that the monthly discount is only supposed to apply to reservations that are a minimum of 28 days.
I'd get straight back to the guests and explain the mistake. Also I would send a note or call Airbnb. Giving a discount should not be detrimental to you, if should encourage longer stays, but you still need to make a profit.