Well, according to their documentation, notices are sent at 20, 10, and 1 day before expiration. (Though in practice, I've only seen the 20 and 10 day notices myself, even for certificates that I didn't renew, but much of my testing is in the staging environment.) If by that point it hasn't been renewed, it's more likely to be the case of the certificate not being needed anymore (due to a site no longer being used, or due to a new certificate that exists but has more or fewer domain names on it) than the case of somebody got the earlier emails but didn't actually check that their automatic renewal was working right.
I guess I wouldn't really object to an additional notice at 0 days left or day-after-expiry, but I don't really think it would help many people. I'd rather effort on improving expiry notification be spent on making the notices better (like not sending at all if new certificates exist for the each name in the expiring cert even if it's not quite the exact same list of names). And even better would be improving the popular ACME clients such that if they were failing renewing day after day to be able to better alert server administrators.