I have run across the same thing in many stories and am wondering if there is something I am missing.
The issue has to do with text messages in the stories. In many stories, one of the characters is texting another one and ends their text exchange with 'J' ... Have tried to figure out why but no clue. It is not their initial nor does it seem to be short for 'joking' from contextual clues. If it just happened in one story or with one author, I would classify it as accidental inclusion or unique to them, but I have seen it in at least 20+ stories with more than half a dozen different authors.
So... is there a reason for ending text messages in stories with 'J'?