The challenge was not unclear on that test case
log10(1000) = 3 isn't some weird constraint that the challenge imposed, or unexpected behavior that the OP ruled on where it could have gone one of two ways. It's simply the correct answer to the challenge with its existing specification. Adding it to the test cases doesn't cause answers to become valid or stop being valid, it's just a useful edge-case that happens to show that many existing answers were never truly valid. This isn't a case where the specification was changed due to the OP not liking some answers or the like.
Any invalid answer needs an individual notice that they are invalid
A user won't know that an answer was discovered as invalid because of comments on the challenge or any meta post. The method I know of is to comment on any post which is discovered to not meet the spec, or even propose an edit which resolves the issue. Just using age, or age since the issue was first discovered on one answer, is not useful because the user doesn't know that it happened. I don't necessarily have some good resolution for determining who is responsible for that check, but we shouldn't delete answers made in good faith without giving the user a chance to fix it (though, if something does get deleted, the user still has an opportunity to fix it afterwards).
Deleting isn't the only fix
Sometimes an answer on other challenges is constrained to some seemingly-arbitrary upper bound of input, often having to do with the language's number representations. I am not opposed to answers that are "only guaranteed to work up to inputs of 1000 due to floating point precision errors" in a roughly similar manner. It would be prudent to establish a firm upper bound for it, i.e. you should know with certainty what the lowest number pair that produces incorrect output is. In reality most languages represent numbers in a format with a strict upper bound, or variable precision, and so almost all answers in almost all challenges involving numerical input without upper bounds there is some input that is technically within spec but will cause every answer to break.
(1000,10) isn't a particularly high input to be causing such an error, but as long as answers identify such bounds I don't think they should count as invalid.