Why is a film calling for Taiwan reunification drawing fire in mainland China?

A mainland Chinese film titled "The Belief," which depicts the 1683 Battle of Penghu where the Qing dynasty brought Taiwan under its rule, is facing unexpected criticism from nationalistic voices in mainland China. The film's narrative focuses on Emperor Kangxi's order to Admiral Shi Lang to achieve victory over the Kingdom of Tungning in Taiwan. This historical event, which led to Taiwan becoming part of the Qing dynasty, is now the subject of controversy. The backlash has cast doubt on the film's future, despite its intention to promote reunification.