This is not how this feature works:
Without that option set, a search for "a b" would find all listings that contain "a" in any given word, and "b" in any given word. With the option set, only listings that contain exactly "a b" would be returned.
Hence, searching for "ABC15" with return " ABC152", because it contains exactly " ABC15".
Have your tried search for "ABC15 " (note the space of the 5).