We discussed the behavior with the developers and it seems to be a bug. The same functionality works when the type of the column is string but it brakes when it is a number. Check out the following sample:
We will investigate what is causing this behavior and fix it as soon as possible. As a token of gratitude for reporting the issue to us I have updated your Telerik points.
As a workaround until we fix the issue, you can configure custom data source and return distinct items from your service