Your format string is OK. You simply don't have enough positions to show the whole value. If you would change the value of the numeric indicator to "1234567890" (same 10 digits as it is in your code, but this value is better to see which digits are missed) then you will see that leading "1" isn't shown. It is because you have the number of positions less then it is necessary to show whole value. Simple add positions to show whole value.
All the best,
the Telerik team
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