Relative paths such as those you are using ("doc1.docx" vs. "Macintosh HD:Users: ... :doc1.docx") work in reference to the currently active folder, which in general is the folder last used by Word. These operations change the active folder:
Opening a document
Saving an unnamed document
Saving As an open document
When that happens, relative paths will be relative to the last-used folder. Then, the Referenced Document field no longer finds the files.
As you discovered, simply reopening your main TOC document will correctly set Word's active folder. However, even though you leave it open, changing the active folder gets you back in the same spot.
The solution is to close the TOC document, reopen it and immediately update the TOC.
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