It was implied in the Christmas chapter when He Tian comes back and Mo is drunk saying that Tian just disappeared
https://www.mangago.zone/read-manga/19_days/bt/br_chapter-134197/pg-1/
Good catch but it was a mistranslation. The raws says “中学” which is middle school. So far we have no evidence that Tian stayed until high school.
Now of course Tian might have left during high school, but the chapters prior to the time skip were already alluding to Tian’s school transfer, so I’m assuming this is when he left for a long period of time.
He left at around 15. Now they are supposed to be out of college soon (22), given that Xixi is applying for jobs. Mo waited and waited without knowing when or whether he will be back.
He taped out Tian's face but never scribbled over it so that he could still look at Tian's face when he missed him too much. The box was taped up over and over again, showing that he did indeed open and tape it up multiple times.
Tian's face twisted in agony upon realizing how much he hurt Mo though ┗( T﹏T )┛