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I think you just changed the cell format for "Mth Date" column as "MMM" from "Format Cells > Number > Custom". That is why while you apply the Slicer on "Mth Date" it shows a different date.
To change it properly you need to apply TEXT Function. Assume that, your "CRM Date" is in A2 cell and "Mth Date" is in B2, then use this formula in B2 cell and copy down to convert "Mth Date" as month name:
=TEXT(A2, "MMM")
Now refresh the Pivot Table, and you will see like below slicer:
Another cause would be the "DD/MM/YYYY" format you are using in current workbook. Does your PC really supports this date format? Generally it should be "MM/DD/YYYY". So please check it.
Hope it will work.