Found my old iPhone XS case so I ran out to the garage to try it out. Charged fine regardless where I put the phone with the case on it. I moved it around intermittently over the course of 5 minutes and had no issues. I don’t get the same results at all with either my iPhone 12 Pro or my wife’s iPhone 12 Pro. In my case I’m not suspecting there is anything wrong with the charger or it would theoretically exhibit the same behavior regardless of the phone that is placed on the charger. In this case both 12 Pro’s behave the same and the XS works fine.