Trade-in mysteriously canceled by Apple after sending trade-in kit

6 days ago I filled out the trade-in info for a 27" iMac 5k, and Apple shipped me a trade-in shipping kit. Now when I checked my status, they have mysteriously canceled the trade-in with an error-like message "TRADEUP_CANCELED_63_STDT" and Apple Support has no idea why, and cannot reinstate the trade-in. When I go to the web page to initiate a new trade-in, I just get a "This service not available" error. All of this is feeling quite like Apple reneging on a trade-in commitment, now that I have already purchased the new device. Frankly, it's a pretty unpleasant feeling.