I had a similar problem last year with all 3 phones I have in the IUP program.
The phones were traded in at different times over a 2 month period, in each case I was accused of sending back an empty box. I demanded an investigation, after about a week on each case, the phones were located, I was told they were "misplaced".
This year I am making recordings of shipping the return phones. In an uninterrupted video, I recorded powering off the phone, placing it in the return box, clearly showing the shipping label, and placing it in the pickup box. I have completed 1 of the 3 trade-in this week and thankfully no issues, but glad I had the video just in case.