Just watched this film yesterday after reading an article about this movie being popular in North Korea(People watch it secretly, of course. All South Korean media is banned, but I heard that people who get caught watching this movie is especially more punished than people who get caught watching other South Korean movies)
I think part of the reason why North Korean government might be more persistent on this movie is because the life in the Japanese Occupation looks better than current North Korea.
People going to theaters to watch a movie, freedom to wear whatever you want, freedom of religion, better economic quality etc.
Less freedom of speech in South Korean standard, but much much more freedom of speech than today's North Korea.
Many of the older generation in North Korea apparently told their children that life in Japanese occupation was happier according to North Korean defectors.
Just watched the 2nd film that came out on 1991 and I now understand another reason why North Korean government is upset about the movie.
The movie mentions the name "Kim Il-Sung" as a traitor and murderer of Kim Jwa-Jin, an independence fighter and father of Kim Du-Han.
This Kim Il-Sung(born 1897) is not the same person as North Korea's Dear Leader(born 1912). He just happens to have the same name.
North Korean citizens who watch this movie might mistake this Kim Il-Sung as their Dear Leader, who they have been brainwashed as the greatest hero in the entire history of Korea.
Unfortunately, the government can't clarify and never clarified to their citizens that this Kim Il-Sung is not the same Kim Il-Sung as their Dear Leader.
3 reasons in my opinion
1. By clarifying, they would have to mention the name of the movie, and it'll make people who have never heard about the movie want to watch it
2. They have brainwashed their people that their leader Kim Il-Sung is "one-and-only" and no one else ever had the same name as him. Clarifying that there was a different Kim Il-Sung would be contradictory to their years of brainwashing
3. They have brainwashed their people that their leader Kim Il-Sung was the only existing indepence fighter who fought for Korea's indepdence and he was the person who liberated entire Korean peninusla by defeating the Japenese imperial army. They would have to mention real freedom fighter's name(Kim Jwa-Jin), who has done more for independence than their fraud leader Kim Il-Sung ever did. People would then find out there were other freedom fighters beside their leader and would make people curious about real history.