I went to the store with screenshots of my purchase history and whatever I could find. I never got a proper answer from Steinberg on who has to authorize the Crossgrade copy, but it seems that it’s in the hands of the reseller to verify that you own the product you are cross-grading from.
I don’t remember exactly how I found the purchase history, but I believe it was through the App Store on Mac. Or through iTunes (which I believe no longer exists on the newest MacOS).
The box that I purchased (Cubase 10 Cross-grade) contained an activate code which I could use without any further verification process.