To reproduce use the following code:  

radDateTimePicker1.Format = DateTimePickerFormat.Custom;
radDateTimePicker1.CustomFormat = "MM/dd/yyyy hh:mm tt";
(this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.Provider as MaskDateTimeProvider).AutoSelectNextPart = true;

now, in the first part, type in "10" and you will be auto moved to the second part. Then, press back, to go back to the first part, and try to type in "01" (entering January). The result is that when you press "0" you are moved to the second part, hence "01" cannot be entered.