Long ago, Microsoft admitted that this was a bug, and it's still a bug today.

Try this: With the document opened in Microsoft Word, select Save As to bring up the file dialog box. To the left of the Save button, you should see a little drop-down menu called "Tools." Click on that and select General Options... That will bring up the General Options dialog box.

In that dialog box, if the option called "Read-only Recommended" is checked, then uncheck it.