Hi,
I'm not sure whether this is a nwb or react-octicon problem.
I'm using react-octicon inside an nwb project, which is displayed inside a electron application.
In electron, urls are usually loaded via the file:// protocol, which causes the fonts to not show up since the browser is trying to load them from /build/<hash> which is the os root :)
Generating only relative filenames url(<hash>.<extension>) would work for both environments.