A few more interesting tests I ran:

1. Instead of using a specific Spring Data JPARepository for each sub-type, I tried using one for the parent class - i.e.:

   public interface DataSourceResourceRepository extends JPARepository<DataSourceResource, Long> {
   }
   
   @Inject private DataSourceResourceRepository settingsRepo;
   
   settingRepo.save(...some CsvDataSourceResource...);
   settingRepo.save(...some DataBaseSourceResource...);
   

   with same result - i.e., failure

2. I tried making the parent class a concrete rather than abstract class - same result (i.e., failure)