Hi! Try using Google docs. I think this will simplify your task, and there are no similar problems with Google Docs. In addition, you can use templates like
https://docsandslides.com/googledocs/cover-letters-templates/ [docsandslides.com] to create documents that will also facilitate your workflow. If you do not want to use Google Docs, I think you can try to remove and reinstall the Microsoft office package since what you describe looks like a strange program failure, and reinstalling the program should solve the problem.