‘About face’ is an expression used in the American military. It literally means to face the opposite direction. In this sentence the words ‘about face’ mean that one person changed their mind and decided not to go with the other person.
你決定不跟我一道去，實在違背了我們當初的約定。
About face 是美國軍隊中的用語：「向後轉」。本句指二人已講好一同前往某地方，不料其中一人改變心意，決定不跟另一位同行了。