Setting the active attribute of a paper-button inserted into a slot using setAttribute requires calling it twice before it is updated. about paper-button HOT 2 OPEN

This seems to be the case with other properties such as raised too, but not properties not defined in the paper-button such as my-attribute.

Instead of setting active twice, one can set raised once and then active, at which point raised will be false and active will be true. I.e. the first time any property defined in paper-button is set, it is not updated. After that, all properties are immediately updated.

from paper-button.

That happens because these properties do reflectToAttribute and have a default value, e.g. see active property.
I'd suggest to set the property instead of the attribute, e.g.

ready() {
  super.ready();
  const paperButton = this.querySelector('paper-button');
  paperButton.active = true;
  console.log(paperButton.hasAttribute('active')); // true \o/
}

from paper-button.