@Heather2084
Another Host posted in the CC about the same problem.
The fact that it says previous discounts is a good sign. You can go into your listing in private viewing mode as a guest yourself and see if the 20% discount is still being applied on a reservation. When I checked it, it doesn't show strike through pricing or a discount on the reservation page on Aug 6-7:


If it is still being applied, or you want to get a refund from Airbnb for the incorrectly applied 20% discount for the 4th booking, you would have to contact Airbnb about it, but I'm afraid you won't get a correct answer, or different answers from different CS reps, but you can try. Might not be worth the effort. It does sound like a bug with Airbnb and may be tied to the new software release 05/13/25.