Symptom

officeatwork add-ins seem to have missing icons in various places.

Cause

officeatwork 365 add-ins use Microsoft UI Fabric as a component to display icons. To do this UI Fabric offers a dedicated font that is embedded into our add-ins (web pages). Some (mainly larger) organizations block embedded fonts on web pages.

Resolution

Enable web fonts. In IE on Windows you could enable web fonts (Allow font downloads) in the Internet security zone settings of IE or you could add *.officeatwork365.com to the list of trusted sites in IE. Any of these options should enable the web fonts for IE on Windows.

More information

Learn more about blocking or enabling web fonts in this blog post: Link