I replaced the date in H1 with the text string " May" and the change occurred. ( reformatting to "mmmm" did nothing)
So I suspect it has something to do with the fact you used text instead of a date when adding your new column, the changing it to a date
I'm not savvy enough in charting to help you any further.
Sorry

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Thank you for attempting to resolve Pecoflyer! I agree that the issue has something to do with how the chart is reading the format.

The issue has to lie within how the chart is interpreting the data as after recreating the chart it displays correctly.

If anyone has any thoughts please let me know.