This might be a limitation of JavaScript. 𪇆 is U+2A1C6 in Unicode, and codepoints above U+FFFF (which need four bytes) are coded via UTF-16 surrogates. And that … sucks.
The canonical bug for that in WordPress is #13590 Inserting a 4-byte UTF-8 character truncates data. It was closed as maybe later, because fixing it would require a change in the database encoding that WordPress’ current schema just cannot handle.
You could try the plugin TinyMCE Chinese Convert and switch to Simplified Chinese to get around that.