The difference is that the certificate authority validates DNSSEC and most of the resolvers you tested with do not, or do not enforce it strictly.
DNSSEC is a chain of signatures. Your zone signs its records, the parent zone publishes a fingerprint of your signing key, and a validating resolver walks that chain from the root down. If any link is broken, a validating resolver returns nothing at all — not a wrong answer, not the unsigned answer, a hard failure. That is the whole point of it.
A non-validating resolver ignores the signatures and hands you the records. So the site works for you, works for your phone, works for most online lookup tools, and is invisible to anyone whose resolver checks.
That is why the error looks contradictory. You and the authority are asking the same question and only one of you is checking the answer's signature.