Do you have an ACF field named “post_title”, if not and this is referring to the WP post_title then this is not a meta value and it cannot be searched with a meta query.
When WP does a search is breaks the search string down into individual words and adds multiple “LIKE” clauses, 1 for each word.
search string = "this is a search"
LIKE "%this%"
LIKE "%is%"
LIKE "%a%"
LIKE "%search%"
Like queries require enclosing in % characters.
