I’ll tell you what happened

They use a special device that Apple designed themselves that’s used to heat and then remove the display from the frame enough to open the phone up, and someone probably forgot to take it off or just wasn’t paying attention, and it melted a component in the side button.

How do I know this? A mixture of having five+ years of phone experience, and I’ve also had this exact thing happen. They were supposed to replace the display on my 12 pro, but they broke the volume down button with the machine that’s supposed to open it up. They had to replace my entire phone for free. Good times.