Why did Zhang Tan and Su Lan break up at Dad College?

Dad College is an online drama adapted from comics. In the play, Zhang Tan and Su Lan broke up because Zhang Tan's ex-girlfriend came to school and had some troubles, which led Su Lan to misunderstand Zhang Tan's behavior and think that he still had feelings for his ex-girlfriend, so he finally chose to break up. But Zhang Tan finally explained the truth and made them get back together. This also shows that their feelings and trust have experienced some twists and turns, but they have finally been repaired and deepened.