paper-chip-input validation doesn't work

[alkunjo]

As it says right here https://github.com/fabbricadigitale/paper-chip#input-field-feature
We can do validation to paper-chip-input manually. Somehow when I use it inside a shadow-root of a polymer element, it can not be validated as the browser says

Here is my code for the paper-chip-input

<paper-item id="newRC">
   <paper-chip-input id="rcListAndInput" autocomplete placeholder="Choose categories" required></paper-chip-input>
</paper-item>

And here is where I try to validate it manually using a polymer observer on rrc-new.html line 143

checkValidateProperty(submitted){
  if(submitted == true)
    this.$.rcNewForm.validate()
}

Is there anything I have wronged to do? Or it seems to be a bug?

[leodido]

@alelb could you have a look here?