Saving as docx in Writer

Hello Chaps
Sometimes we are forced to save our Writer documents as if they were Word docs.
Up to now this has worked well, but now it produces a zip file.
What is going on?
Laurence

A docx file IS a zip file. Are you saying that LibreOffice actually save a
file with the zip extension, or just that it show up as zip in your file
manager? The former would be very surprising, while the later can happen,
especially on systems where the content of a file is used to determine it's
type instead of the extension.​