The price increase/decrease percentage does not work when on Smart Pricing. So first Smart Pricing must be turned of for the specific time period and a fixed price must be entered.
If not on Smart Pricing, then the discount can also be achieved by setting a fixed price in the same way.
(It seems when the rule set is created, it can not determine if Smart Pricing is active or not).
Maybe a programming challenge for Airbnb to make your wish come true !