I think it's strange that Noctua is including their old 120 mm fan NF-P12 instead of the improved NF-P12 FLX which has better airflow at lower noise.
There is no NF-P12 FLX.
There is a Noctua NF-S12B FLX 120mm Fan. But that doesn't work well on heatsinks. It doesn't work well with the type of high back pressure that heat sink fans face in that type of application.
When the NF-P12 runs into back pressure it is able to keep the air flowing even against the impedance. In such circumstances, the NF-S12B FLX keeps turning, but fails to move much air.