Unicode handling has slightly changed with version 5 of Font Awesome. Now you also have to set the font-weight. But I cannot find such a property in for example the textStyle property. So my question are you supporting Font Awesome 5?
You have to set the fontstyle in the font property correctly then it works (so it had nothing to do with zoomcharts in detail).E.g.: font: '900 17px "Font Awesome 5 Free"'
font: '900 17px "Font Awesome 5 Free"'