The way the name of the month is written depends on the Culture settings of the Report/TextBox. Either the Report or the TextBox with this date uses a culture in which the December month is written this way.
I have attached two images demonstrating how the date will be displayed when the default en-US culture is used.
To resolve the problem in your report, you may either change the Culture setting of the report itself or of the TextBox.Culture property.
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