Ace was stumped by both Thomas and the system's notification. He never expected the situation would turn out to be like this just by meeting with Thomas.
It wasn't like he had forgotten the Thief Promise he unknowingly triggered that day. But he never expected it would be triggered now of all time.
Nevertheless, by how Thomas was kneeling in front of him and his storm-like feelings of grief, pain, and hatred, he knew something had happened after he left, and it seemed to be completely changed the happy-go-lucky Thomas that he knew.
"There is no need to kneel, Brother Thomas. We are no strangers. Aren't I here to help you in the first place? Tell me what you need." Ace said with a smile before he turned back to being White.
Since the system was telling him to listen to Thomas, he mused the mission would be triggered after Thomas revealed his plight and there wasn't any danger to him as well.