– Does this happen on any search phrase?
– Are there any non-post types choses as results (like taxonomies under the General Options -> Sources 2 panel, or users, attachments) ?
– What is the limit set under the General Options -> Limits panel?
Can you please try what happens in cases when the:
1. Search override is turned off
2. and when the override is turned on and the method is set to GET
Let me know if anything changes in these cases.