mlkem768x25519-sha256 is one of two post quantum algorithms SSH supports but are not currently supported in Bitbucket.
As of OpenSSH 10.1 a warning is now displayed if a quantum safe algorithm (like this one) is not supported by the server:
** WARNING: connection is not using a post-quantum key exchange algorithm. ** This session may be vulnerable to "store now, decrypt later" attacks. ** The server may need to be upgraded. See https://openssh.com/pq.html