It seems that you are using the SameRowRecurringTask SDK example. I tested the specified code snippet and there isn't anything wrong with using the RecurrenceTask in an array. The problem is that the same object is added multiple times in the RadGanttView. You can notice that the in the 2nd for loop "RecurrenceTask_Array[i]" is added to the 1st and 2nd GanttTask. Instead, you need to create new instances of the RecurrenceTask objects.
Please, find the mentioned SDK example updated to demonstrate how you can populate the RecurrenceTasks. Do check it out and let me know, if it helps.