As Paul has already said, Word's master documents are not reliable.

A possible alternative, if you really don't want to keep all the content in a single *.docx file, is to bring in content from external files with INCLUDETEXT fields. You can insert such fields via the Insert File dialog box (Insert | Object | Text from File). To create the field, cselect a file and then click the arrow on the Insert button and choose Insert as Link.

INCLUDETEXT fields must be updated manually after the source file has been edited.

Conversely, you can edit the field result and save the changes back to the source file with Ctrl+Shift+F7.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional