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 b 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.