Reader/lurker, not one who posts often, but seek advice.

Needed to fly ATL-PVG, business, in May. Delta wanted >$6K for the direct flight. MU had a flight YYZ-PVG for about $3K. Combine with a roundtrip ATL-YYZ and I paid about $3.5K total.

Fly the route, miles credit normally, except for the return PVG-YYZ.
ATL-YYZ, RT, 550 MQDs and 2218 MQMs
YYZ-PVG, outbound, 2843 MQDs and 10661 MQMs

But for return, PVG-YYZ, Delta awards 0 miles, 0 MQMs, 0 MQDs = Reason, website says: Mileage credit denied by partner - activity not eligible for mileage credit.

How is this possible? It is an advertised, paid, I class ticket on MU.

What am I missing? Is Delta simply mad I did not purchase their over-priced direct flight?

Thanks, everyone.