Such format is not supported and the validation of the field will fail. This is due to the fact that month and year alone could not be used for creating a Date object. You can easily test this in the browser's console by parsing a string to a Date object with only the month and the year:
As you will notice, the above will not create a Date object. The same apples to the field validation. If you need to select only a month, you might consider using a simple DropDownList with the months.
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