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

The future of a mainland Chinese film calling for reunification with Taiwan has been thrown into uncertainty as it faces a surprising backlash from nationalistic voices.
The Belief tells the story of how in 1683, Emperor Kangxi of the Qing dynasty ordered Shi Lang, the admiral of the Fujian navy, to lead the Qing army to a major victory over the Kingdom of Tungning in Taiwan, which considered itself part of the then-overthrown Ming dynasty.
The battle, known as the Battle of Penghu, ultimately...