Using the git version of ssh-audit, curve25519-sha256 is currently flagged as an unknown algorithm whereas [email protected] is correctly identified.

Looking at the sshd_config man page for OpenSSH 7.6p1, both are valid for KexAlgorithms. The default sequence is listed as

curve25519-sha256,[email protected],
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1

Some software such as SecureCRT 8.5.2 require the usage of curve25519-sha256 instead of [email protected]