ForwardAgent?

Could it be that since the Certificate is not in the IdentityAgent, forwarding does not work?

OpenSSH_7.9p1, OpenSSL 1.1.1k-freebsd 24 Aug 2021
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 1: Applying options for *
debug1: Connecting to targethost-nfs [172.16.254.156] port 22.
debug1: fd 3 clearing O_NONBLOCK
debug1: Connection established.
debug1: identity file /home/user/.ssh/id_rsa type -1
debug1: identity file /home/user/.ssh/id_rsa-cert type -1
debug1: identity file /home/user/.ssh/id_dsa type -1
debug1: identity file /home/user/.ssh/id_dsa-cert type -1
debug1: identity file /home/user/.ssh/id_ecdsa type -1
debug1: identity file /home/user/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/user/.ssh/id_ed25519 type -1
debug1: identity file /home/user/.ssh/id_ed25519-cert type -1
debug1: identity file /home/user/.ssh/id_xmss type -1
debug1: identity file /home/user/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_7.9 FreeBSD-20200214
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9 FreeBSD-20200214
debug1: match: OpenSSH_7.9 FreeBSD-20200214 pat OpenSSH* compat 0x04000000
debug1: Authenticating to targethost-nfs:22 as 'user'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: mailto:curve25519-sha256@libssh.org
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: mailto:chacha20-poly1305@openssh.com MAC: compression: none
debug1: kex: client->server cipher: mailto:chacha20-poly1305@openssh.com MAC: compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:9DZQE+gXw6BolwutR3GPutxfqKzlRSvjimNf9DOrXHw
DNS lookup error: general failure
debug1: Host 'targethost-nfs' is known and matches the ECDSA host key.
debug1: Found key in /home/user/.ssh/known_hosts:1
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey after 134217728 blocks
debug1: Will attempt key: id_rsa_sam RSA SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU agent
debug1: Will attempt key: /home/user/.ssh/id_rsa
debug1: Will attempt key: /home/user/.ssh/id_dsa
debug1: Will attempt key: /home/user/.ssh/id_ecdsa
debug1: Will attempt key: /home/user/.ssh/id_ed25519
debug1: Will attempt key: /home/user/.ssh/id_xmss
debug1: SSH2_MSG_EXT_INFO received
debug1: Fssh_kex_input_ext_info: server-sig-algs=
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering public key: id_rsa_sam RSA SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU agent
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: Trying private key: /home/user/.ssh/id_rsa
debug1: Trying private key: /home/user/.ssh/id_dsa
debug1: Trying private key: /home/user/.ssh/id_ecdsa
debug1: Trying private key: /home/user/.ssh/id_ed25519
debug1: Trying private key: /home/user/.ssh/id_xmss
debug1: Next authentication method: keyboard-interactive