I'm working message log in our app. We want to show diff of changes any user made on text posts. Those posts may include emoji characters but diff_match_patch replaces those characters with � character (but only if they changed in the text).

The behaviour can be reproduced here:
https://neil.fraser.name/software/diff_match_patch/demos/diff.html

Paste 😉and 😀into textareas and compute the diff.

Why is this happening?