We don't fully render rich content in news feed snippets, so what displays is the raw text of the message, including the raw smilies e.g. :giggle: rather than and the raw emoji characters e.g. 🌈 rather than
The weird black characters are unfortunately how the browser or OS sometimes renders some characters which is a quirk I've noticed on macOS too as well as Windows but XF doesn't have any control over how they display.
This is a benefit of rendering the emojis to a standard set of images like we do in posts but, again, we don't attempt to do that for text snippets so ultimately what you have reported is expected.