I get the same issue, but it is not consistent. Sometimes it happens, sometimes it doesn't. It is very very gay though. I only ever use the enter key to make my new lines. Maybe I accidentally have the shift still pressed from a previous capital letter (Constant struggles in the shoutbox has made my typing speed pretty fast). But I am pretty sure that isn't the case.
Because I have had certain posts where it keeps happening every singly time I press backspace, and I keep having to go back and re-hitting enter to add the line break back in, and I am sure I am not somehow accidentally hitting shift every time.
But it shouldn't be happening anyway, and it is very very gay, and this is the only time I have ever encountered this issue on the entire internet, and it has happened several times as I have typed this post, and I am definitely just using enter.
I do use a java script blocker (although I allow most of the scripts on the codex), but I wanted to mention it in case that is impacting it in some way.