You should be able to reproduce the problem using the following code:
DemoPartyDetails Editor Template
When running this code, I can click the button to add a new party, the editor template appears as expected and I press the 'Male' button, which populates the gender field with "M".
I then fill in the other two fields with generic "Test" and "Person" data before confirming the entry and then pressing submit.
What I find when I place a breakpoint at the controller however is that the model being passed through has the gender set to null. The first name and surname ("Test" and "Person") are fully populated however. The randomData is also fully populated as expected.