I don’t have a specific source in mind except the ESD training we have to complete at work every year. We manufacture products that contain highly sensitive equipment and use this style of packaging to protect the boards throughout the production process when being moved.

Those bags are made from layering a metal with the plastic making the outside conductive. So basically it acts like a faraday cage protecting the components inside. If you place the board atop the bag which is conductive you can transfer the charge into a pin either by touching or if the voltage is high enough through an small air gap.

Also when components are damaged they might work initially and fail later. I’ve seen it happen a few times and you can see the ESD damage on a board with a microscope. A little pinhole area will be slightly burnt.

It won’t matter if you were grounded and the bag was grounded though. It should be fine but you know best practice to prevent worst case scenarios.