I am quite familiar with the error. Please read my post carefully. I point this measurement observation out as this is a STRONG HINT that something is NOT correct!
In any case, I measured their "standards" independent and I found the 50 ohm load at 58.5 + j 11.5 ohm at 900 MHz.
The short and open are 412 pH and 190 fF respective.
I suspect no polynomial L or C coefficients are loaded to a table in the nanoVNA to reflect these deviations.
