Search API Solr Autocomplete no results when more than x Nodes indexed
RTFM - Today i finally found this: https://www.drupal.org/node/1999392#hidden-variables


  search_api_solr_autocomplete_max_occurrences
      By default, keywords that occur in more than 90% of results are ignored for
      autocomplete suggestions. This setting lets you modify that behaviour by
      providing your own ratio. Use 1 or greater to use all suggestions. Defaults to 0.9.


This makes perfect sense since 90% of my total 25 nodes is 22.5.
I just set in a custom module:

variable_set('search_api_solr_autocomplete_max_occurrences', 0);


And it works.