Unfortunately, after looking at this again, I realized I misunderstood this in the documentation: The translation uses a summary of the report as the evaluation context . The “summary” does not include all of the report fields, but only a limited sub-set of mostly special system-populated fields.
However, to just include some fixed text (e.g. “Case Management”) along with the name of the contact (the value that is shown by default when the subject_key is not set) you could use: Case Management {{subject.value}}. Alternatively, if you are not using the case_id functionality from the registration transition one work-around for loading custom data from the report is to save the data you want in a case_id in the form. Then, you can access that value in the subject_key via {{case_id}}…