I’ve already raised an issue for this - but thought it might help others to mention it here.
In 3.1.1-pl - when adding a new User and changing the Password Method to anything other than Let MODX generate a password - the radio button for Show the new password on screen becomes unchecked.
In this state, saving the user is unsuccessful - but no notification is presented on screen so it feels like save was successful.
Making sure the “Show the new password on screen” radio button in checked before saving solves the issue.
Also occurs while editing a User.