I had this week a price reduction of -30 % for a booking to one of my apartments too. As it turned out, there has been three "promotions" stacked:
Mobile rate -10 %
Genius: -10 % and
Last minute -10 %
This is far too much for me. It seems, I have to delete one of the promotions for good, although it is a tough one, because we have quite a lot of guests from each group. The other way would be to set a ceiling on the resultant of possible discounts ie. "not more than 20 %" This option would be good, as it will allow individual promotions to run, without a need to adjust or delete them every time. Hosts could rest assure that none of the stacks would cross the maximum they had set. I would like to recommend this option to Booking.com management.