I've used Word for around 25 years and have never seen such behaviour, so I can't suggest a design reason for it. Changing Styles & editing other portions of the document are unlikely to be contributing factors. More likely it's document corruption or a faulty Office installation.

Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

Similarly, in case it's a problem with the Office installation, you could try repairing it (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change).

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]