If no query-handler is defined ez Publish chosses one as seen in "Final query parameters sent to Solr backend".

For example when using the wildcard '*' within the query the standard-handler is
choosen. However, when the wildcard '?' is used the handler ezpublish is choosen,
but this handler only supports + (required) and - (excluding) and not wildcard-queries,
so there are no results.