That is currently per design. Old reports need to be reprocessed in order to get data for it.
You can trigger that on the command line (see https://piwik.org/faq/custom-reports/faq_25265/) or use the invalidate reports plugin to invalidate reports, reprocessing should then happen automatically in the browser or when the cron job runs.

We need to change some stuff in Piwik itself in order to be able to add new reports for already archived periods. We hope to make that possible in the near future, so plugins like custom reports can add their reports automatically without reprocessing all reports.