I have already replied to the support ticket that was opened by you on the matter, so I paste my answer below for the rest of the community:
Range Bar chart type is similar like the bar chart type - displays discrete data, so you can have only a single item from a series over a single category. The only difference come in the from and to properties of the range bar. That being said having multiple items from the same series over a single x-axis category is not reliable.
You may also find useful the RadGantt
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