We will try to reproduce this on our side. Before that, could you clarify the following points?
If you decline to reboot at the end of the upgrade and re-enable ssh (sudo systemctl enable ssh), on reboot the connection is still refused.
Does that mean that, if you enable SSH after the upgrade and do not reboot, you cannot connect via SSH at that specific point?
If so, it would be nice if you could check your SSH configuration (/etc/ssh/sshd_config), as it has probably been overridden.
If you restore sshd_config and restart SSH, does it work?
IMPORTANT: You must check sshd_config, not ssh_config.
Also, could you check that SSH is enabled?
sudo systemctl status ssh.service
Update - If you replace the ssh_config with the version by package maintainer you can connect via terminal or ssh client - but oddly it breaks console over web in Lightsail/AWS.
Note that AWS Lightsail probably requires some SSH keys to be available in "~/.ssh/authorized_keys", and either it is getting overridden, or the authentication mechanism was changed (e.g. from SSH key to password authentication).