Querying the first page gives me the first 20 results: https://src.fedoraproject.org/api/0/group/java-maint-sig?per_page=20&projects=true&page=1
All later pages (determined by the "next" link in the pagination) return zero results, making it currently impossible to query for all a group's packages.
This is fixed by https://pagure.io/pagure/pull-request/5112# .
@pingou , any chance of getting minor release and deployment pretty please :) ?