A stray "J" I encountered years ago: a certain client's support tickets would often end with a single "J", which was a little confusing as it was not one of their name initials. After a brief investigation, the original email source contained this:
\n is ^J (ctrl-J) so there's some logic there, but I wonder if something has been lost in the message. I'd guess it either displays ^J or an inverse-colour J, rather than just a plain lowercase j.
Also possible that the j is a red herring and just some random character that's always there. Pasting a URL containing a newline into most browsers just truncates it at the newline, regardless of how much text is after. I only know this from occasionally copying links from a terminal window where the copy somehow added newlines every 80 characters (even though copying this way normally works fine). I'd have to copy the URL with newlines into a text editor, remove the new lines and copy again to be able to paste it.
Some tool or library is interpreting the newline as two characters (as you note), and then a subsequent step is removing unprintable characters. Things like this used to frequently happen in shells, Perl, PHP, and so on.
also the same 'j' found in words like 'jujuism', 'jejunities', and 'bejeezus', also by a magical coincidence the same one in most Latin fonts, and even some random text strings such as 'pj$4'
?
[0]: https://tex.stackexchange.com/a/64848/270600
[1]: https://superuser.com/q/212874
[2]: https://en.wikipedia.org/wiki/Caret_notation
Also possible that the j is a red herring and just some random character that's always there. Pasting a URL containing a newline into most browsers just truncates it at the newline, regardless of how much text is after. I only know this from occasionally copying links from a terminal window where the copy somehow added newlines every 80 characters (even though copying this way normally works fine). I'd have to copy the URL with newlines into a text editor, remove the new lines and copy again to be able to paste it.
The same 'j' as vi uses for 'hjkl'. https://vi.stackexchange.com/questions/42426/why-did-vi-use-...
Specifically, J is the 10th letter of the alphabet and therefore ctrl-J is code for ascii 10. Same reason ctrl-D sends EOF and ctrl-I sends tab.