Mac Numbers - cells lose formatting

I have a very straightforward spreadsheet, and have formatted each column to suit, e.g. date, number to 2 decimals places, align left, or centre, etc.

Every so often (quite regularly, I'd say every 15 - 20 rows) the formatting becomes undone. If I enter a date such as 12/9, it will stay as 12/9, rather than change to 12/09/2023, which is what I want. In other columns, the centre alignment reverts to align right; in numbers, the two decimal places I've pre-formatted for that column's cells will change to just one decimal place. Etc, etc.

I correct this by clicking the very top of each column, one at at time, reformat as desired, then all is well. But at some point before long, it all becomes undone again. Why is this? What am I doing wrong, if anything? This becoming irritating. I hope there's a fix, because I don't want to move to Excel.

Thanks for any responses...