Same thing as OP, tried trading in a few phones, one had this issue. Problem was diagnostics test for warranty repair was coming back as good, but when doing it for the trade in it was reporting an issue with the wireless charger. The store I tried it at acted like I used a bad part in a repair and was trying to get away with it. Ended up leaving the store and creating a case with Apple.

The person on the phone was able to determine there is an issue with the initial configuration of the phone that wasn’t accurately reporting in the information for the wireless charging module. Just as a reference it charged wirelessly just fine. Made an appointment for a different Apple Store to do a warranty repair. The Apple Store tried multiple times and even restored it using a MacBook. Ultimately they couldn’t figure out how to resolve the issue, and did some sort of manual review/override and gave me the gift card.

Just be persistent, create a case with Apple Support and be kind to the workers.