I started splitting my React application into several bundles using React.lazy. For some reason though, if I do this the Handsontable starts to render funny.
So I actually figured it out. We had the old react-handsontable package installed along with new @handsontable/react.
Since CSS was removed from the new React wrapper (@handsontable/react), I needed to add import 'handsontable/dist/handsontable.full.css'; to get the Handsontable styling.