Unexpected change to date format

Recently number/currency formatted columns are changing to date, not the whole column just the new entry. I will enter 495 for example and get a date. I change the format and the correct amount is displayed. It has happen on different spreadsheet over the last few days.
I tried deleting direct formatting, which fixes it but then randomly in other pleace I see it has happened again.
Is there a setting I need to look at?

from an upgrade ?

This is the guide - How to use the Ask site? - #3 by Hrbrgr