This question is really interesting, hahaha, what can I say? From my personal point of view, it may be that the expressive force of the work is insufficient when shooting, which confuses the audience, but in fact, the screenwriter has some arrangements, and there may be two reasons.

First of all, I was originally Cheng's apprentice. This journey is too narrow-minded. After being defeated by Huo Yuanjia with "Fan Wei Quan", he was very angry with himself, and then dumped the pot on Huo Yuanjia, causing Marco to take revenge on Huo Yuanjia. Considering how good the kung fu of such a master is, it makes sense to say that Zhen Chen was mediocre at first.

Later, Chen Zhen really wanted to get back at Huo Yuanjia, so he pretended to worship Huo Yuanjia as his teacher and assassinated Huo Yuanjia. Although he doesn't really want to be a teacher, Huo Yuanjia, as a martial arts master, must have taught such excellent young people to practice martial arts. Therefore, under the guidance of Huo Yuanjia, Zhen Chen has made progress in martial arts, which is the first reason why his kung fu has become better.

Secondly, Zhen Chen failed to assassinate Huo Yuanjia many times, but he was moved by Huo Yuanjia's generosity. He sincerely regards him as a teacher, and from his life, he feels the purpose of China Wushu: keeping fit and saving the country and people. So I think the moment he put on the Chinese tunic suit also represented the change of Zhen Chen's realm, not just for himself, but for a higher ideological realm. Then, just like the "epiphany" often said in China Wushu, Kung Fu has also been promoted to a higher level, just like being reborn. This is the second reason for his great progress in martial arts.

Of course, these changes may be better understood if the film and television are more subtle. Of course, it is also my own opinion, for reference only.