Dear Paul,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and to my knowledge, the behavior you are encountering in the SharePoint Online Document Library is not an expected behavior because we need to be able to edit the properties of the files in the library when it’s created or uploaded.
Since you mentioned the problem happens only with a specific document, I would like to suggest you access the Document Library in the InPrivate or Incognito window of the browser and see whether you are able to add the properties to the problematic Word document.
If still not, can you please move the document to your OneDrive and move back the document to the Document Library again to check the behavior?
Right-click on the problematic Document> Move to> My Files> Move here> Now go to your OneDrive> Right-click on the Document> Move to> Select the site under Quick Access (if site is not listed, click on More Place under Quick Access)> Choose the location and click Move here> Now check the behavior.
If still the same issue, please create a new Word document and copy the document from the problematic document and paste it into the newly created Word document to see whether you are able to add the properties.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia