It can be hard to identify the exact cause of the issue. Still, generally speaking, this indicates a problem with the "behind the scenes" (XML) components that Excel uses to determine what the sheet should look like, including column sort logic, formatting, default settings, etc. There are numerous possible causes. However, I previously saw it when I unintentionally changed the template. Try copying all the crucial information, pasting just the values into a fresh workbook or Google docs letter templates free, and then formatting everything once again.