foulborn emperors vigilance bugged

hello, i think there is a bug with foulborn emperors vigilance.
i cant seem to increase the quality above 20% using tainted armour scraps?shouldn't i be able to get it past 20% to 40%?
since its an unscalable value shouldn't it be a constant +20qual then you can add on top of it to 40%?but it just erases the quality on the unscalable part