Custom mandatory fields in the user story are not captured in the template
Please keep in mind that this is not considered a defect.
Any Rally Template supports three child item fields: Name, Description, and Notes. If additional fields on a story are set to mandatory, the user should still receive a warning flare about the needed fields because they will not be set.
So, if you have a custom mandatory field in the user story that the template does not recognize (other than name, description, and notes), the user story is not created using this template.