What I observed is that the entries in known_hosts created by ssh that are not supported contain a string ecdsa-sha2-nistp256 instead of ssh-rsa.
Hashed entries with ssh-rsa are supported.

readelf -d /usr/bin/x2goclient
does not show a dependency on libcrypto which contains the elliptic curve cryptography functions while
readlef -d /usr/bin/ssh
shows such a dependency.

Best regards

Heinrich Schuchardt