I encountered the same issue on my client’s site. Turns out it was Relevanssi plugin that was intercepting all search queries. I had to go into relevanssi_prevent_default_request function and allow achievements related queries to come through without being captured by Relevanssi.
You can simply hook into relevanssi_prevent_default_request filter and return false when the query is related to achievements, i.e. when $query->query_vars['post_type'] is either achievement or dpa_*.
