S21 gain figures

[Alle1307]

I am trying to do some tuning of band filters. I am using the nanovna-H with release 0.6.0 and in combination with nanovna-saver.
It came to me that the filter characteristics were not quite what was expected and therefor I tested some cables to be used. Indeed some cables were old and gave a lot of attenuation and so I made some new hyperflex cables with new BNC connectors.

After calibration I expected to see a flat line of 0 dB on the S21 gain chart for the through setting for the whatever attached cables. The result should be straight line, but it is not.

On the contrary I see a wavy chart through the spectrum dependent on the type and length of cable this looks like a mismatch between on the S21 port. (See the attached figure for the submitted sma cable.) It seems that the correction figures after calibration are not applied correctly.

Is this normal, a hardware fault or a software issue?