First, I understand where you're coming from. :) But there's a reason. Mo and He Tian are more popular with readers, and their slow-burn romance is finally coming to life! People are excited about them and Mo's character development. :D
Also, when we get back to the main plot, it will focus more on Jian and Zhengxi, because we are learning more about Jian's background. The author is human and has their own reasons for doing these short stories. Historically Old Xian has done shorts for Chinese festivals, holidays, and celebrations. And they might be planning the story before they draw it. To make sure it's perfect for us, the reader!
It's okay to feel disappointed. However, do not accuse any creator of not caring about something. There may be reasons you aren't aware of and it creates a negative mood that may dissuade a creator from continuing their work.
"I miss seeing more of Jian and Zhengxi D: I hope we see more of them soon when the story continues." is a way to get your same thoughts across in a more positive way.
Sorry for the long rant and for my 2 cents. Just trying to be helpful.
