The second module was modified with the same modifications. Looks OK.
10dB attenuator at RF2126 module input improves gain flatness, so seems like the signal generator source impedance is not perfectly equal to 50 Ohm. See the log for the second module modification results.
No attenuator at the input, only a 20dB attenuator at the spectrum analyzer input
10dB attenuator at the input of the RF2126 module
Short/long cable comparison
20dB attenuator at the input of the spectrum analyzer.
The cable length is 150mm (Wuerth RG174/U 65503503515303)from the generator to the input of the module.
Comparison between 150mm and approx. 1m (RG174) lengths on the output of the module.