I investigated the scenario and the RadGridView behavior is the expected one. 

When there are more than 1000 distinct values for a specific property only the first 1000 sorted values will be displayed in the filtering popup by default. When the DistinctValuesDisplayMode of a column is Tree only these 1000 values will be taken and grouped. 

There are several ways to modify this behavior:

1. Increase the DefaultDistinctValuesCount of the RadGridView.

2. Use the DistinctValuesLoading event, which allows for modifying the distinct values that a column uses for filtering.

3. When a column is bound to a DateTime value, its FilterMemberPath can be set to "DateTimeProperty.Date" as suggested in the following article so that only the date part of the DateTime is considered while collecting the distinct values.

With the above information in mind, I will proceed with setting the status of this item to "Declined". Of course, feel free to contact us, if you have any additional questions. 

Regards,
Vladimir Stoyanov
Progress Telerik