I have stepped through the code and I can see that the iconCls is correct (ux-gridfilter-text-icon) when the inputItem is added to the menu in StringFilter.js (row 59) but it doesn't stick for some reason.
The css (GridFilters.css) is loaded and the header style is set correctly when a filter is applied (default bold/italic).
RangeMenu class works as expected (displaying icons >, < and =).