It turns out that the order of metadata is significant, e.g. with metadata such as dc.contributor.author and dc.contributor.editor.

DSpace respects metadata order, e.g. see dc.contributor.editor for http://www.ssoar.info/ssoar/handle/document/5847 and on the OAI-PMH interface, http://www.ssoar.info/OAIHandler/request?verb=GetRecord&identifier=oai:gesis.izsoz.de:document/5847&metadataPrefix=xoai .

Changing from Set<Metadatum> to List<Metadatum> should require lots of rewriting in the DDA codebase, but Eclispe should help to accomplish it.