Data validation does not validate values if they are not set using the cell editor. Pasting or auto-fill of invalid values will not produce data validation warning.
After further investigation by the development team, we can confirm that this behavior is by design, and is exactly the same as in industry-standard apps like MS Excel. The item will be declined. If the issue is still relevant to you, please contact Telerik Support via the ticketing system, and we will do our best to provide workaround for your specific scenario.