It was probably a friend of his. When u have friend u sometimes let em borrow ur phone to do who knows what. My friends would ask for my phone and I would just give it to them without hesitation so maybe that happened to sangjae, probably a friend of his used it for a few minutes and sent that message to later delete it in sangjae's phone
like it or not, some people/ur friend (especially jerk like sangjae's friend) would sometime peek when u unlock ur phone and secretly knew ur password. it did happen to me before. i had a friend that peek into my chats w/o me realizing (they took my phone while i occupied w other things) and when i was looking for my phone, i saw they had it in their hand, reading my chats. i seriously think that was rude and quickly change my password afterwards. i would say u r lucky if u never encountered such situation.
so to answer ur questions;
1. it's logic and not impossible.
2. sangjae might not realize it because the friend might have delete the chat (one sidedly) before giving it back.
Yea i know sharing phone isnt uncommon. But only with close friends. And why would a close friend send a message and then delete it. And let's say the close friend is just a fake ass bitch and tries to create drama, what he did would have easily been found out by most people....so it's pretty stupid.
Sorry if it wasn't clear but it's the whole thing altogether that seems weird.
yea, but i dont think his reason was to create drama. its more like he want to cut their relationship because he hate to see sangjae always prioritize hangyul when they don't even went to same school. we can also see it clearly, that hangyul is the "close friend" to sangjae and the other guy is just a "friend".
tbh i don't think its uncommon.. but i do understand what you mean by weird, because their friendship was ruined and that fake guy get away with his fake chats w/o anyone knowing. but if we view on the fact that hangyul suddenly shut sangjae down without telling him why, it make sense.
well, this is my pov on this situation, well maybe because i understood hangyul who tries to not being a bother to other people. this traits of his was the reason why this "stupid msg" could easily ruin their friendship, because we can see sangjae actually did try to talk things out but hangyul didn't want to. its because he was already sad bc of his parents, and when he finally try to open up, this fake friend hit him up w such msgs. it probably hurts him psychologically that he got scared to open up again. (︶︿︶)

Well.....how come someone was able to use Sangjae's phone? (Password??)
And then, that person sent that msg to Hangyul and Sangjae never noticed the message ?????
I guess this is just basic BL drama logic but i hope theres some more explanation