I tried using both this method to set the value in assigned group and assignee:
issue.setCustomFieldValue(assignedGroup_cField, assignedGroup)
issue.setAssignee(defaultAssignee)
or
def changeHolder = new DefaultIssueChangeHolder()
assignedGroup_cField.updateValue(null,issue,new ModifiedValue(issue.getCustomFieldValue(assignedGroup_cField), assignedGroup), changeHolder)In history it is showing the correct value of assigned group, even in log it is showing the correct value but in the issue the assigned group is not updated.