There are two ways artifacts mainly appear.

Card is running with high temperature over a extended period, so the artifacts can be permanent when you push the card to a certain degree.

Or what i believe is most likely the case and i see on my cards as well (22x of them) 1150/1600 will simply cause this, after a extended period of mining. Tune down core clock! (i even get artifacts at 1050 then again i am running all my cards at 1040mv for best Hash:Watt ratio) reboot required, artifacts don't go away just by down-clocking the card.