@Fran-Diox I have confirmed the issue on RC17 as well. I created a new project with RC17, just added:

<ons-input placeholder="00"></ons-input>

No custom CSS and the placeholder goes to the top left of the input. I can only guess that something else is happening in the default CSS. I inspected the object and it gets display: none so I assume something else is overriding it.

Edit: Confirmed this only affects material and not flat.