Did the trick! Thank you so much, you saved my bacon! Waiting for MDM to see it is not stressed enough really (In my case Addigy). Join the device in ABM, go back to MDM and wait for it to see it, then reboot. I'm going to help my client set up future purchases with the apple store business team. Then they pop up in ABM right away.
The only other place I came across this, other than your post, was buried in a Jamf article. Well done and thanks again!