ok I may sound crazy but I feel like someone around Yahwi is betraying him especially (IM SAD SAYING THIS) pil choongman which is his blonde haired friend. Yahwi's grandpa told mr.na to keep a close eye on Y and we know how J went to meet y's grandpa meaning he knows (?) What's going on between them and how we saw that shadow figure spying on them. On chapter 46 when j said he was going to the bathroom pil looked at him with a "hm" kinda look now here's my theory:
Either pil went to ask j something or talk to him or he used that as an excuse to report something to y's grandpa. The reason I'm saying that is because we didn't see pil come out the bathroom with j or when he was about to leave the restaurant. We only saw him when he was heading out with yahwi.
Or he was just smoking a cigarette or taking a caca idk.