I tried to extend my stay for 2 days and the pricing is 40% more than if I made a new booking. My original booking is at a weekly 20% discount but the calculation appears to be repricing the whole trip with no discount and subtracting what I already paid!
Few points,
1. Airbnb says this is not a system issue.
2. Airbnb says no discount will apply to any extension. I checked this by trying to extend for a week and yes I can only get the discount if i make a new booking and not extend.