Just wanted to mention all info come from nasaspaceflight.com.
the Ku can provide data at greater ranges much than 135K, it's just that operationally the measurements aren't used until inside of 135K as the range measurements are corrupted beyond 142K. The 7K difference is to be conservative and ensure no bad data is processed in the nav system.
As well, the Ku can provide quality data down to 100 ft minimum. For ISS flights, the angle measurements become noisy at close ranges, but the range and range rate values are solid. Again, operationally, it's just easier to stop processing all measurements.