The news items helper news_item_teaser will use the custom teaser if it is present. https://github.com/refinery/refinerycms-news/blob/master/app/helpers/refinery/news/items_helper.rb#L21-L23 However, the news extension doesn't support customer teasers. I think this is a copy paste error as the blog extension does support custom teasers. https://github.com/refinery/refinerycms-blog/blob/master/app/helpers/refinery/blog/posts_helper.rb#L13-L15

The easiest fix would be to remove the incorrect lines from the helper. However, a better solution could be to add support for custom teasers. This should be fairly easy because the rest of what's needed for them could just be copied from the blog extension as well.