packet_write_wait: Connection to UNKNOWN port only with ProxyCommand

At work I have to connect through a gateway to reach the machines where our code is stored. My laptop is setup with Ubuntu 18.04. When I connect to the remote machine in two steps:

laptop: $ ssh login@gateway
gateway: $ ssh login@work_machine

it works fine. However, I have set my config file for ssh so that I can connect directly from my laptop, with the following content:

Host gateway
  Hostname gateway
  User login

Host machine_from_laptop
  Hostname work_machine
  User login
  Proxycommand ssh bdemoulin@gateway netcat -w1 %h %p %r

Now I can connect to the remote machine with:

ssh machine_from_laptop

But the problem, after 2-3 seconds, I am disconnected with the error message:

packet_write_wait: Connection to UNKNOWN port 65535

I have played with ServerAliveInterval or ServerAliveCountMax and TCPKeepAlive, with no effect... Also I don't have any access to server side parameters. What bothers me is that the disconnection only appears when I use the config file...

Does someone has any idea of what is wrong with the config ?