It's because the values are getting set by Form_builders set_field_values method in the module controller and overwrites the values set in your form_fields method.

How we get around it in FUEL is to create a new field call 'new_password' and then use model hooks (e.g. on_before_save()) to set it to the password value.

Ok, thanks!

And how about some other field?
Do I have to overwrite the field with other?

Could you provide an example of what you are asking to help me visualize? Thanks.