Firstly I do not have
File > Options > Custom Dictionaries
File > Options >[Recent | OneDrive | This PC | Add a place | Browse]

Secondly when I insalled office 2016, I set the languge to English (UK). So it is not the built in American custom dictionary. I have tested it out with words spelt diferently in Amercan.

Do I have add an additional dictionary as well as the American and UK dictionary.
Also this is a home computer not connected to any network.


Also when you add to dictionary, you definatly do not add to custom.dic. Word creates a seperate file for words added.