Seems like something wonky is going on with line breaks.
If I press enter twice while composing a message, I usually end up with a super-wide break, like so.
If I only press enter once, I end up with no break at all.
If I go back and forth and back and forth including some edits after pressing the preview button, I can sometimes get it to look how I'm trying, like so, but it's painful. (edit: and like this, it almost always requires editing the post, at which point my extra breaks have magically appeared and I can delete them)