I am also having the same problem after upgrading to 8.1.8.
In the Search Everything settings under Search Everything Basic Configuration I only have “Search every custom field checked”. I don’t have “Search every attachment checked”. However, attachments show up in the search results.
There has been a slight change in the SQL search everything generates, with some brackets removed. This seems to mean all post types are returned where it should just be posts of type “post”.
Thanks.