My recommendation is to roll the thumb over and down so the pad of the thumb hovers over the palm of the hand. Then at full draw, the thumb fits UNDER the jaw bone with a side of face anchor.
IF the thumb is sticking up, or behind the ear/neck, that can lever your string fingers out away from your face and frankly adds to the tension in the hand and wrist for most. Many shoot OK with the thumb up but in coaching circles, we try to avoid it.
Arne