Details matter in trying to solve your problem - if you can't be accurate in describing your issue and don't want to supply a sanitised sample doc then there isn't much we can do to help.

Your story varies a bit - "so far the only document to exhibit this behavior" vs "However, older docx that I have on file, written by someone else, also exhibit this behavior" & "is less pronounced when working with .doc documents"

Normal.dot implies the template is not Word 2007 in its format. I would expect Word 2007 to be using Normal.dotm
You can rename the Normal to anything you like. If a file with the 'right' template name is not found in the User Templates folder it will be regenerated automatically when opening Word. Word itself is not responsible for a file named Bnormal vanishing from a folder - that is more likely to be a PEBKAC issue or possibly caused by a macro which may have been inadvertently run.

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Andrew Lockton
Chrysalis Design, Melbourne Australia