Hi,
I have implemented this use case in my PDI, This issue is arising because of Mandatory of Assignment group that extending from TASK table. And I can Dictionary has Mandatory false for Assignment Group
1) If Assignment group is not mandatory in TASK dictionary then Assignment Rule is working fine.
Screenshots: 

2) If Assignment group is mandatory in Dictionary level and tried with onLoad Client script with method below is not working g_form.setMandatory('assignment_group',false);

3) I tried with flow designer as well it's working fine for non mandatory of assignment group and not for mandatory assignment group while creation of problem.