Thanks @jkuester; adding a calculate field case_id in the app form with the values that I require concat(${outbreak_name}, ' > ', ${patient_name}) solved it. Used together with the subject_key.
form.properties.json
{
"title": "Case Investigation",
"subject_key": "report.title.case_investigation"
}
messages-en.properties
report.title.case_investigation = {{case_id}}
Results
Note: This workaround doesn’t apply to the reports card on the patient’s profile page
[Edit]
I noticed that; in the reports card on a profile, report list subtitles only show sometimes. Still investigating this will create it as an issue once certain.
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Although I can use conditional cards as suggested by @michael here Customize report title in report list (for app forms) - #6 by michael