When saving a password (e.g : smtp password) from the admin interface, it is not saved.

I think this may come from org.ametys.plugins.core.impl.model.type.xml.PasswordXMLElementType.fromJSONForClient(Object)

    @Override
    public String fromJSONForClient(Object json)
    {
        // Cannot get password from JSON;
        return null;
    }

When saving, this is executed, and then "null" is passed.