The field should only update if some field in the record was modified. This could be as subtle as a set field step that sets a field to the value that it already has, but just the change in layouts by itself will not result in the field updating to show a new date.
Look for script triggers that modify a field in the record.
Keep in mind that two layouts can refer to the same table so something on either layout might be modifying the record.
Hey Phil, turns out your were right. I have a script that updates every product number (not the auto serial number I use as a primary key) in every record every time one record's product number is updated. So thanks for pointing that out.
For anyone else interested,
I needed a field that would update a modification time stamp only when certain predetermined fields are changed. This post solved my