Hairline Cracking Only Appears When Stoop Is Wet

Dear All, I just renovated the entire facade of our brownstone. The team took the facade off to the brick, put on an undercoating of cement, waited 3 full weeks then applied the brownstone. They worked hard and diligently. I am watering the stone daily. I’ve noticed hairline cracking just before the stone completely dries. It is not noticeable when dry. I am very concerned this condition will only worsen as we go through the seasons, ultimately resulting in large cracking and flaking. I have withheld final payment, and the team is willing to fix the situation but I think they are stymied. Any advice would be so appreciated. I’ve attached a video of the condition. Many thanks in advance, Kukla