If you watched the series, you will know that Yu Zhen and Zi Tong were together and then broke up in a turbulent way seven years before the events of the series. Zi Tong actually comes back to Yu Zhen because he wants to relieve the burden of their past relationship and let him go on with his life and live happily with Shi Lei. Which makes sense, despite being incredibly annoying (I would have preferred it if Yu Zhen was able to actually go on with his life without his ex having to step in and get involved in everyone's business). As ridiculous as it might sound, Zi Tong gave Yu Zhen and Shi Lei his "blessing" and left. The whole Zi Tong business had already lasted long enough at that point, he got lots of screentime in the series, to the point that both main characters saw him in their dreams. At the end of the series I was already thinking that this character had been shown way more than necessary, so when I saw the special episode I was hoping to get some more moments about Yu Zhen and Shi Lei.

Instead of that, we got an entire new episode where Zi Tong Just. Won't. Leave.
As much as his story with Yu Zhen was important for the plot, it should have been incorporated in the series in some way, or abandoned altogether. If the past is past and it's time to think of the present and the future, then why is it necessary to see everything and more about a past relationship? At some point I just came to the conclusion that someone in the production team really loved the Yu Zhen/Zi Tong couple, so they tried to create more content about the two of them despite their couple not being endgame. I can't think of any other valid reason beside that (or trying to get on my nerves).

I only recommend watching this if you're someone who wants the full picture about Yu Zhen and Zi Tong. If you're looking for sweet moments about the actual couple of the series, prepare yourself to be disappointed.