Activation Lock still active after I sold my Mac.

I owned an iMac Pro for six years, and just sold it. I followed the Apple Support article for how to reset everything. I used the Erase Contents & Settings command and it appeared to hang for an hour or more. So I restarted the Mac. And this time it brought up a black screen with a Network Picker. So I thought it was done, and ready for someone new to pick their network and set it up.

I also checked my iCloud account and verified that this iMac Pro was no longer associated with my Apple ID. The same Apple ID I used on it for six years.

But no. When the buyer got it home, the Activation Lock screen came up for him and it said the iMac was associated with an Apple ID that was not mine (shown only as o*****@icloud.com). That was entirely impossible, as no one else has even touched that Mac and there were no other user accounts by that name.

So we tried over the phone with every combination of my Apple ID and password I've ever used, and of course it eventually bricked the iMac.

I opened an Activation Lock case with Apple Support but there is no indication whatsoever about how long this might take. The form required me to identify my Mac by serial number, which worked. But they also required me to upload my sales receipt that shows the same serial number. The problem is, I bought it directly from the Apple Store and the emailed receipt doesn't show any identifying information.

This is absolutely insane. I've done everything as directed by Apple Support and the UI in Sonoma macOS. Has this happened to you?