Sorry for the clickbait title. When the vertices explode, no way we can fix it (imo). But there is a way to prevent it in the first place so that you guys can spare some headache.
If we look closely at the problem, we could see it only happens around the upper eyelids, although the shape key included the mouth aswell, but the problem never occurs there. So I did several tests and found out what caused the problem. The culprit is the combination of Multires + overlap geometry. When you sculpt over this area, the multires modifier gets confused and messes up the geometry. But you dont get to see that until you close the eyes using shape key. So the shapekey got blamed for revealing the mistake (shapekey could introduce problem but not this one).
So here is how you avoid the artifact: Never sculpt over the upper eyelids when the eyes opens (you could do whatever you want when the eyes closed, there is no overlap geometry so it is safe)
