Rows being deleted unintentionally

Excel 2021:  I have had severl instances when opening a particular workbook, or similar copies, of rows being deleted and the rest in a specific group of rows, being run together.  I have 12 worksheets with months names on the workbook and this is happening on 11 of the sheets.  I have to copy the good sheet, then select the other 11 and paste.  It has been happehning many times this week.