The default font size on the client is not saved explicitly on the Excel file, so the Excel defaults take over (11px Calibri), so upon subsequent import all cells will have that font.

Steps to repro:

    go to https://demos.telerik.com/aspnet-ajax/spreadsheet/examples/import-export/defaultcs.aspx 
    export the file
    import the file

Expected: nothing changes

Actual: all cells are 11px Calibri
Workaround: Set a default font when the cell font is null
 
public override void SaveWorkbook(Workbook workbook)
{
    string path = "C:\\path-to-output.xlsx";
     
    for (int i = 0; i < workbook.Sheets.Count; i++)
    {
        for (int j = 0; j < workbook.Sheets[i].Rows.Count; j++)
        {
            for (int l = 0; l < workbook.Sheets[i].Rows[i].Cells.Count; l++)
            {
                string fontFamily = workbook.Sheets[i].Rows[i].Cells[l].FontFamily;
                workbook.Sheets[i].Rows[i].Cells[l].FontFamily = !string.IsNullOrEmpty(fontFamily) ? fontFamily : "Courier New";
 
                double? fontSize = workbook.Sheets[i].Rows[i].Cells[l].FontSize;
                workbook.Sheets[i].Rows[i].Cells[l].FontSize = fontSize.HasValue ? fontSize : 12;
            }
        }
    }
    workbook.Save(path);
}