omg so sorry for the late response- but essentially, we see hints of mgs resenting he tian for leaving without a word in the “future” episodes. These episodes are made apparent by him having his own apartment, working as a mechanic, etc. meanwhile, the rest of the characters hv moved on with their lives and don’t hang out as much as they used to (granted, they still catch up and hv meals and stuff). it’s also seen with the struggle between he tian and his family that they want to send him off somewhere in the school episodes, with implications that it might happen suddenly and leave mgs resentful.

So remember: what we are reading rn is a super long flashback. Therefore, it was already revealed to us that these four will soon separate and never see each other for a couple years or so. It was already shown that He Tian will leave first, so what most likely happened is He Tian broke the news to Mo GuanShan first that he was leaving them. However, we do see in the chapters alluding to the future/ present time that they reconnect!