Page provider 'publish_space_suggestion' does not work in ES

If we add the pp `publish_space_suggestion` to the list of page providers in `elasticsearch.override.pageproviders` than the search returns nothing when clicking the drop down box to list the sections where we publish a document.

The generated ES query is:

'\{"from":0,"size":40,"query":{"bool":{"must":[{"constant_score":{"filter":{"term":{"ecm:mixinType":{"value":"PublishSpace","boost":1.0}}},"boost":1.0}},\{"match_phrase_prefix":{"dc:title.lowercase":{"query":"","slop":0,"max_expansions":50,"boost":1.0}}}],"adjust_pure_negative":true,"boost":1.0}},"_source":\{"includes":["_id"],"excludes":[]}}'

The problem is the 'match_phrase_prefix' with 'max_expansions:50' => the query wont return anything when we have many docs