FYI, multiple users in our organization have also reported this issue after upgrading to Windows 10 1809. Rollback seems to fix it.
Happening across multiple versions/channels/architectures of Office (2013, 365 / Monthly, Insider / x86, x64).
Opened a support case with Microsoft, they are aware of the issue.
As bad as this bug is, thank goodness it's not just me who is experiencing it! It is indeed present on Microsoft Office 2013, 2016 and 2019 and nothing seems to fix it.
If Microsoft are aware of the issue, then they should release an update as soon as possible. I've been able to circumvent the problem by copy/pasting entire Word documents into Google Docs, then downloading them back as finished .docx files. This is very tedious to do, however, so I am hoping they will get to the bottom of it sooner rather than later!