The ACF relationship field uses the built in search mechanism. If any word you type is in the title or content of the post then it will appear in the list. If the exact phrase appears then it should be closer to the top, but it needs to be exact.
How WP Search Works
ACF only loads a few posts at a time, not sure of the number. If it stops before it has shown all the possibilities then there might be an error during the AJAX request. You might be able to figure out the error by turning on error logging.
