When I tried to generate a font overriding the "👥" symbol (codepoint: 128101), fantasticon seems to change the codepoint to 61697.
The problem seems to be

According to https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/String/fromCharCode
String.fromCharCode() only supports 16-bit codepoints. ES2015 offers a similar function String.fromCodePoint() which does not have that restriction.

When I replace it locally, it seems to work.