if you look at the owners manual it shows that when air directed to the bottom, some comes from the top as well (small arrows top, large on bottom).
i too find it very annoying
one thing i hate is that the temperature from the top vents seems to be just as hot as the bottom. if the air were a bit cooler it wouldnt be bad.
obvioously manually closing the vents solves the issue, but i hate doing that, because i am manually fiddling with the automatic system.
agree that it it is not broken - just an awful design.
have another vehicle with auto climate, when the cabin reaches set temperature, OUTSIDE air come in to the defroster. ouch, on a cold day it is awful. that system is made by Denso, and this was common for all debso systems around the time.
any idea who makes the lexus system? maybe there is a setting to close the top 100% when air directed to the bottom.