I see constant complaints very similar to yours on these discussion forums. And, in fact, the same thing happened to me earlier this year.
Am I correct in assuming you appraised your trade in honestly, and that there was nothing wrong with it when you sent it in? (This was the case with mine).
What I did was to Reject the "updated" trade in value and had my 2016 MBP returned to me. Upon getting the returned Mac back I took it to the Apple Store, as the third party company Apple uses for trade ins said my display was damaged and my trade in was worth $0. The Apple Store tech checked it out, and mentioned they rarely see a 7 year old Mac in pristine condition like mine was. I was given an Apple gift card for a larger amount than my original trade in value, so I was happy. You might try this if Apple Support doesn't help you.