We found around 2% of destinations are missed in routes and destination country according to TI redirect data, could you please help confirm why this happened?

The empty destination (airport, city or country) is caused by MULTI-CITY trips. For multi-city we can’t definitively say what the person’s destination is (e.g. if they travel MEL-SYD-DPS-MEL was their destination SYD, DPS or MEL?). So partners are encouraged to use leg/segment information for every search / redirect to define their own logic in the case of multi-city trips.