Unwanted reformatting of protected cells

I have a number of spreadsheets in which I have columns formatted in a
particular way - e.g. Time to display 00:00:00 or ID formatted to display as
a Number with no decimal places.

All of these cells are protected (so I believe should NOT be allowed to
change without my active involvement in that change).

HOWEVER, after saving the spreadsheet, subsequent opening of the spreadsheet
causes these protected cells to change to DATE format!

I do NOT want this to happen and it takes up too much of my time to
constantly reformat all of the changed columns.

WHAT is causing this to happen and HOW do I prevent it from happening and
HOW to I ensure that a desired and saved formatting is saved correctly to
prevent this issue?