It's just the emoji. Emoji's can be part of the url. So, you have to add the space if you want to render it correctly.
Discussion
But the space is added in other clients when you write in another line (tab+enter) but I think amethyst does not translate a new lane as an space and that's why it "merges" emoji from previous lane with the url from the next lane, that's why the video can be displayed in most of the clients but amethyst