Symptom 

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

Cause 

The caching on the MacOS is preventing the Icons from appearing. 

Resolution 

Clear the Add-In cache. To do this remove all files from within the Data folder on your Mac. You can find the Data folder here: /Users/{your_name_on_the_device}/Library/Containers/com.Microsoft.OsfWebHost/Data/