Hello baohulu,

Sorry for my late reply.

my understanding is like this:
Gmin is related to the port impedance you choose, Zs, and the input impedance at NFmin, Zopt. While NFmin is only the minimal noise figure. NFmin unchanged doesn't mean that Zopt is not changed.

NFmin changes with the input refer current source and the input refer voltage source (and their correlations) in the picture. When the matching network change, if it is noiseless, then NFmin will not change, only Zopt will be shifted. This is because for each frequency, you can always find a shift in source impedance to counter-act the change in the matching network, and arrive at the NFmin. And because your Zopt now change but Zs=50ohms not, your Gmin now changes.