So this is a bug in that Signal TUI he was using? I.e. it mangles newlines in pasted text.
I was poking around with this and I noticed wl-copy has an option to trim newlines. Maybe that's why they added the option but I'm leaning towards gurk being the culprit. wl-copy itself seems to handle newlines ok, at least for me. This works as expected:
echo -e "test\n" | wl-copy
wl-paste
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:
Which renders as a smiley face.