This is a DNSSEC test zone whose DS and DNSKEY records announce both ML-DSA-44 (algorithm 18, per draft-westerbaan-dnssec-mldsa) and ECDSAP256SHA256 (algorithm 13), but the alg 13 RRSIGs are deliberately stripped — only ML-DSA-44 signatures are served.
It is the mirror image of the downgrade zone: a resolver that supports ML-DSA-44 validates this zone via alg 18, while a resolver without ML-DSA-44 support finds no usable signatures and treats it as bogus. If you can see this page through a validating resolver, that resolver validated ML-DSA-44 signatures.
Try: dig +dnssec upgrade.alg18.westerbaan.name A