It might be a bug. I couldn’t see anything in CSS that was causing the extra white space. The only way I was able to take out the space was to use inline css for that specific list item.
If I’m not mistaken, you can have a post that is a combination of markdown and HTML. So, I think your best option right now is to use plain HTML for just the list in your post. By doing that, you can set specific CSS settings for the list items.