Group Fields lose grouping after adding data

[OldDude]

I have a pivot table with a date column first following by assorted numerical data in the remaining columns. I apply a group field to the date to group by months which initially works fine. However, when I add new data the pivot table updates the added entry fine but the grouping by month is lost and the field reverts back to a dd-mmm-yyyy format rather than the mmm format originally selected.

I have tried varying the format options on both the date field as well as the pivot table date column with no success

See screenshots of before and after.