He was stealing a few precious moments to himself. The melee was far from over and he knew his uncle would soon be taking the field but he'd been called back by an urgent message from Polonius who mistrusted speaking of anything secret in a pavilion. Not so long ago he might have called his Master of Whispers paranoid, but now... perhaps Polonius had good reason.
His thoughts elsewhere, Jaehaerys didn't notice the girl until he was almost upon her, but the shine of golden hair in sunlight and the... curve of her was certainly enough to distract the young king.