Honestly, I think this is expected behavior and isn't necessarily a bug. It's basing the preview on character count. I don't think there will be very many cases where someone just has short line after short line, so I don't really see this as a problem.
I believe there already is a character limit, which is how it decides to truncate the preview. It's just that with the inclusion of line formatting, a small amount of characters can go a long way when separated by line breaks.