When I'm adding a new person and have entered a birth and death date, and there's an existing profile for the person with the same birth date and no death date at all, the existing profile doesn't show up in the "compare close matches" section. I can see not showing the other profile if it has a death year that's wildly different (though even then, that may be a case of one profile having the exact date and the other "after DATE"), but when the death date is blank and the birth date matches, I'd expect the profile to show up as a possible match.