Comment 4 for bug 1970264

Here is some more info:

The server runs with an enabled UFW firewall. It was set up via

ufw allow from <IP RANGE HIDDEN> to any port nfs proto tcp

Here is the server output of 'ufw status'

Status: active

To Action From
-- ------ ----
OpenSSH ALLOW Anywhere
2049/tcp ALLOW <IP RANGE HIDDEN>
OpenSSH (v6) ALLOW Anywhere (v6)

Now I realized that the original problem disappears if I disable the firewall on the server. I guess the difference is that now the portmapper call gets through.

However, I want my NFSv4 connection to use port 2049 only, as it is working for me with 20.04 (and older) clients.

@Sergio Durigan Junior
By the firewall observation, it makes sense that your first attempt gives a successful automount. I wonder if the mount was cached somehow on the client side when in a second attempt you "forced nfs-server to use NFSv4 only, edited its systemd service file to not depend on rpcbind, and masked/stopped rpcbind".