Hi Neil,
I have this same question. Did you get a resolution? A guest booked 7 days with me and the 10 percent discount definitely calculated on MY end, so I'm being paid 10 percent less. However, my guest is being charged FULL price.
Airbnb makes it difficult to contact them via email and gives no phone number anywhere on this website. I don't understand what they mean by they offer 24/7 support, since they're difficult to reach directly. Perhaps they mean this community right here. It's very frustrating.
I've removed discounts from my listing, since it's obviously not working. I'd rather just reimburse my guest directly than Airbnb keep this discounted amount from me and my guest.
These much worse experiences I'm reading on here are freaking me out and making me think twice about continuing.