Information loss in the Page Curve?

Using the von Neumann entropy definition, the pure states have zero entropy and we have the full information about the system. My understanding is that the whole universe should be in a pure state and unitarily evolves.

But in Page Curve, which describes the entropy change during black hole evaporation, the entropy increases first then decreases to the original value. Is this entropy increase contradicts the fact that the universe should be unitarily evolving all the time? And why can we regain the information (decrease the entropy) at the end?