error valve simulation from 300 to 4 bar for nitrogen and methane

[EvenSol]

Describe the bug
The basic program is working - with the exception of the throttling valve (reduction from 300 to 4 bar). I noticed that the drop in temperature due to the JT effect is not always consistent with the pressure drop.

I made the short program attached only to test the valve, to compare enthalpies of the inlet and outlet stream. It seems that, whenever the temperature is dropped below -20°C in the inlet, the throttling process is no longer isenthalpic. What can explain this?

I am using Peng Robinson EoS, and simulating the same valve conditions using Hysys with PR, I do get an isenthalpic process.