Comment 12 for bug 1812247

Steps for a workaround by disabling gnome-keyring's SSH agent component, tested in Ubuntu 18.04.1:

# disable GNOME Keyring's SSH Agent, cf.
# https://askubuntu.com/a/585212/43108 + https://askubuntu.com/a/607563/43108
mkdir ~/.config/autostart
cp /etc/xdg/autostart/gnome-keyring-ssh.desktop ~/.config/autostart
echo Hidden=true >> ~/.config/autostart/gnome-keyring-ssh.desktop

# add autostart .desktop for plain SSH Agent
cat > ~/.config/autostart/ssh-agent.desktop <<EOF
[Desktop Entry]
Type=Application
Name=SSH Key Agent
Exec=/usr/bin/ssh-agent
OnlyShowIn=GNOME;Unity;MATE;
X-GNOME-Autostart-Phase=PreDisplayServer
X-GNOME-AutoRestart=false
X-GNOME-Autostart-Notify=true
EOF

After a reboot `set | egrep SSH` should return something similar to
SSH_AGENT_PID=986
SSH_AUTH_SOCK=/tmp/ssh-AaTsPiP5tTcj/agent.852