If you're used to use LXC container on Proxmox, you may have realised that something is messed up out of the box. Thankfully, the fix is quick and easy !
Out of the box, it looks like the SSH service does not read the config file, especially the "sshd_config" one. It appears that this functionnality is broken down by the ssh.socket configuration.
To resolve this issue, we have to basically hide the ssh socket service and disabling / reenabling the proper ssh service.
The fix hold in these four commands :
systemctl mask ssh.socket
systemctl mask sshd.socket
systemctl disable sshd
systemctl enable ssh
Now, restart your container and the problem is fixed !