Excel – Sort & filter buttons are (suddenly) not available in Excel table

These numbers look like typical "sequential serial number" format of the Date function, meaning the number of the day.

The cell that displays "MONTH yyyy" is formatted as a date, probably a custom date format since this is not a standard display format. Or maybe you have used the Group By option of the Pivot Table and have chosen "Group by Month".

If you wish to change the display format of a cell, see Microsoft's Format a date the way you want.

We need to know more about your datasheets to give better advice.

I had the same problem for one column, it used to set forecolor of one column to red for each new row I added to the table.

To resolve this click on the header of a column then right click and choose "Format Cells" option and set the desired format. After this new row will use this format for selected column. You can specify the format for other / all columns like this.