>>You could save the "changed" document to another name, then compare the two documents to see if there actually are any differences: https://www.google.ca/search?q=compare+2+word+documents
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>When I made a test by constructing a basic document, I saw that the situation did not happen. It appears this is subject to happen on complex document or based on a specific formatting which might apply.

In some of my standard letters I use date(), I guess this is why I experience the same as you.