This doesn’t seem to happen all the time, but is tricky when it does. Below, I pasted in a direct link to a tweet. In the editor, when I paste in the link, the following appears, where the URL of a downloaded image appears immediately after the link:
Below here is where the link actually was pasted - you should see the rendered results of the link in the edit window:
Note that this happens if you try to put the link inside of code ticks, too. On other occasions, the OneBox seems to appear as expected, but I cannot figure out what the difference is. The bug happens about 50% of the time.