Cannot ignore that the system is noisy, because this is the meaning of the TIS measurement.
May be possible that the sensitivity to get worse with higher antenna gain (antenna pick up more noise from the system), and sensitivity became better with lower antenna gain. But could happen in the other way too, because is never a linear relation between these parameters.
If you want to test the antenna gain and its efficiency vs frequency, shut down the entire system, and do the measurements just for the antenna itself.