It could very well be a bug. I see Bob Jones helped you. He has been a Microsoft MVP for years and I would trust his advice. Since he mentioned he's never seen this before, it indicates to me it's a bug.
The only other suggestion I could suggest would be to set language.
• | Open a Word document. Click on Word > Preferences |
• | Under the Authoring & Proofing Tools, select Spelling and Grammar |
• | Click on the Dictionaries button next to the Custom Dictionary drop down. |
• | Select a language (US English or any other language you want to use) |
• | Click on OK. Exit word. |
• | Restart Word |
• | Try the adding a new word to dictionary. |
If this fails...
I suggest you test in a new User. This will quickly tell you if the problem is system wide or if it's your User's folder that contains the problem.
You can use the Guest User or create a new User for testing.
CREATE A NEW USER
Go to System Preferences --> Create a New User in Users & Groups.
Switch to the New User by logging out/in or use Fast User Switching.
Only default Apple apps will be in the Dock in the new User. Go to Applications to open Microsoft Word.
Do you still see the issue?
If yes, then the problem is with your base files. (this would indicate a bug in Word)
If no, then the problem is in your User's folder.