Are you opening a template that is predefined? It may be that your default template got corrupted. You may have to create them again.
Open one, select all cells and remove any formatting then resave the document.
Excel uses Book.xltx as the basis for new workbooks and Sheet.xltx as the basis for new worksheets.
If you go to one of the cells in question, right click the cell and go to format cells. Take a look and see if any of the options are not what you are expecting. You may have a corrupted template or something else going on. IF possible try opening on another computer to determine if it’s the file or the program that may have the issue.