It has been mentioned that users are having problems with version 16.9 and higher of Office. Office for the Mac has undergone dramatic changes in Office 2016. There have been a number of issues for people with the High Sierra OS. Developers are trying hard to make the Mac and Windows versions have the same or equivalent features. They are making real progress. However, fixing one thing sometimes breaks others.

You can use the User Feedback feature to let them know if they've broken something. There may be a fix coming soon. I’d suggest checking for updates - it may take several tries for Office to recognize that there is an update, so try checking a few times.

Here is the link about progress on Word:
https://answers.microsoft.com/en-us/...1-cce785db2004

In the meantime, I suggest using a previous version until it is fixed.

To do that, you’ll need to uninstall the current version, download the old version, and install it.

Here are the instructions on how to uninstall for Mac:
https://support.office.com/en-us/art...d-b73dc1a8cae3.

Here is the link to install the latest stable version:
https://support.office.com/en-us/art...7#bkmk_current.

Here is a link to allow you to choose an earlier release:
https://support.office.com/en-us/art...0-529000d3d20f.

Here is a link to UserVoice to let the developers know what is causing you problems:
https://word.uservoice.com/forums/304942-word-for-mac.