I have recently updated my Libre Office install to 6.4.6.2 and now all my existing files load the date cells with a “Number Field” of MM/DD/YY instead of a “Date Field” DD/MM/YY
If I change the cell settings then they seem to stay after this.
My defaults are set as following
Interface English UK
Location English Australia
Currency AUD
Date Pattern D/M/Y ; D/M