I don't think it's a tattoo but makeup or a temporary tattoo, then. Because in chapter 15 when we get a look at Jieun and Sangwoo's relationship she doesn't have it. And considering what she (or her body) has been through, it having faded should be normal.
It's not a real tattoo, it's either paint or temporary tattoo that she put on her face for the party event/singing performance. If you read the chapter where she and Sangwoo performed together on stage carefully again, you'll see some of the girls in the audience had this kind of paint/temporary tattoo on their faces too.
The Jieun girl has a heart tattoo but it changed. In chapter 19 it is a full heart and in chapter 21 it is a broken heart. Wonder why.