Okay I just tested the query directly in solr and it's working. But this is initially a filter triggered by a facet so it is a classic use case of eZ Find as I declared the attribute as string in eZ Find.
Is this an issue ? Do I need to post this to issues.ez.no ?
If I understood ok the class ezfeZPSolrQueryBuilder should take into consideration if DatatypeMapFilter to check if it is a string and add quotes around filter value ?