Ah, OK, so it's Google Chrome, but the rich text editor is CKEditor running in a Chrome extension. If I copy directly from that editor and paste into an RTF file, or into the named clipboard in the preferences panel, the formatting is retained. It is the action of running that command in KM that appears not to work.
I can't provide a URL for an example as this is an extension that is not public (internal company tool), but I could record a quick screencast to demo it happening tomorrow. Any suggestions for things I can try to include in the vide by way of diagnosis?