
The first chapters in which he pops up?
I went back and checked.
http://www.mangago.zone/read-manga/19_days/bt/311113/Ch43/1/
This is when the story officially starts (before this there was bits and pieces of their daily lives together(?) but now we got an official story) with the two main characters being reunited. Jian Yi shows up at Zhan Zheng Xi's apartment bloody, bruised, and beaten wearing his high school uniform and book bag.
http://www.mangago.zone/read-manga/19_days/bt/311115/Ch45/1/
We get the background on the characters. They grew up together and went through middle school but on their second day of high school Jian Yi disappeared.
http://www.mangago.zone/read-manga/19_days/bt/311116/Ch46/1/
We get the story of the 'mafia boss' father and how he was taken to an island?
http://www.mangago.zone/read-manga/19_days/bt/286692/Ch1/48/
They are adults and we learn that Jian Yi is back to high school (or taking the equivalent of the GED's to get a high school diploma?) and Zhan Zheng Xi is a college student. It is suggested that Jian Yi is living with Zhan Zheng Xi now.
http://www.mangago.zone/read-manga/19_days/bt/311121/Ch51/1/
Then suddenly we get the story of them being middle school students? Which we've been following ever since. I think we are slowly being caught up to when Jian Yi disappeared as I think they are 3rd years in middle school now?

why do i get the feelings that he tian is somehow involved with the mafia/jian yi disappearance? (if we r following thia timeline) (⊙…⊙ )

why do i get the feelings that he tian is somehow involved with the mafia/jian yi disappearance? (if we r following this timeline) (⊙…⊙ )

Except Jian Yi was gone for several years. He disappeared his second day of high school and returned like 3 or 4 years later when Zhan Zheng Xi was in college.
Then the story suddenly goes back to their middle school years. Because there is never a mention of Jian Yi disappearing and they are both living with their parents (as the sleepover chapters show) when the first chapters have them as roommates in Zhan Zheng Xi's apartment.
I was re-reading, and I realized, is what we're reading right now the flashback before he went missing in highschool, or is it the afterwards he went missing in high school? Because if it's the flashback, wouldn't that mean that they're middle schoolers right now?