Cannot change root password

Hi everyone, I just created a new Digital Ocean droplet from Forge. I'm trying to update root password but it doesn't seem to work:

• from ssh command line I switched to root, using default forge password (sudo -i).
• using passwd command I entered my new password twice. At the end I receive the "passwd: password updated successfully" message.
• next time I log in, the only accepted root password is always the old default password.
• I even tryied to reset the password from DO dashboard: I received a new password vie email, but the only valid and accepted password is still the old one!

What am I doing wrong??