call Amex to resolve
I had this same problem. Technically, it's not a problem (according to Delta) because you need to tell them which account you want the MQM's posted to--since the MQM's are transferable. There is some way you can log onto delta.com and fix it, but it's very hard to find. I called Delta first, they were no help, then called the number on the back of the Amex Business Reserve card and they fixed it for me.