It looks like the SSH key agent protocol has changed since{{ OpenSSH-7.8p1}}, indeed when connecting I see on the terminal:

$ ssh remotehost
agent key RSA SHA256:... returned incorrect signature type 

Localhost has openssh-7.9_p1, remotehost has openssh-7.8_p1. Since my SSH keys are handled by gnome-keyring, I guess the bug is there. See also https://www.illumos.org/issues/9953.