Reduced trade in value

I recently traded in my iPhone X so I could upgrade to the iPhone 12 Pro. There was obviously a trade-in special, so I jumped all over it. Since then, I've gotten the new phone and mailed in my old one, however I received a notification that the credit was reduced because Face ID is not working. The issue I have is that the iPhone X that I mailed in was barely a month old, as it was a replacement device I received after breaking the screen on my original device. The Face ID worked (because it was a new device) when I mailed it in, but conveniently no longer works after testing. I guess at the end of all this, my question is what can I do, if anything, to refute this claim that the Face ID no longer works. I only upgraded my phone so soon after replacing my last one because of the trade-in special, which apparently I'm missing out on anyway. Thank you in advance!