is mixed cabin not allowed for M&M bookings?

I booked YYZ-ORD-HND-PVG over the phone as mixed cabin UA(X)+NH(I)+NH(I). Everything was fine on the call, and I was able to select seats with the booking reference # provided over the phone. But after I got an email saying to call in. First agent said it was billing issue so updated CC. But ticket still wasn't issued and so I called in again. 2nd agent is saying ticketing says it's invalid itinerary because you can't do mixed cabin and that it can't be priced?

Is this a valid itinerary?

Edit: 3rd agent said even though it was confirmed, ticketing wouldn't complete payment / issue ticket because account too new. Keep getting different answers whenever I call