While Tian Xian and Xiu Mei became engaged, their relationship wasn't officially announced or acknowledged yet. The reasoning was simple.
They decided to wait until Tian Xian had gained sufficient fame and reputation before making their engagement public. If the Xiu Family were to announce the engagement of their prized jewel to a common soldier during a time of war with the Seven Heavenly Order, they risked ridicule and mockery.
While Tian Xian had gained fame as a prodigy within the Xiu Family, this knowledge was kept internal, and few outside their circle were aware of his talents.
"Tian Xian, our situation with the Seven Heavenly Order has finally reached the point of war. I will send you into battle so you can acquire experience and make a name for yourself. Do you object?" Emperor Xiu asked him.