To fix it I researched Vector that @user85421 told me about, I ended up finding the synchronizedList class.

What I have done is modify both; the Arraylist results and the destination one ret to this type.

  // Old one List<InformeFacturacionAsociadoDto> ret = new ArrayList<>();
  List<InformeFacturacionProcesado> results = null;
  results = informeFacturacionDAO.getListInformeFactAsociadosV2(cliJurs, centros, desde, hasta, podologos, tipoPrescriptor, prescriptor, servicios, conveniosBusqueda);

   //NEW CODE
  List<InformeFacturacionAsociadoDto> retS = Collections.synchronizedList(new ArrayList<InformeFacturacionAsociadoDto>());
  List<InformeFacturacionProcesado> resultsS = Collections.synchronizedList(results);

This way no null appears and no data is lost.