I booked MF discount business I/J LHR-AMS-XMN-HKG rtn on the MF website (paid with AMEX to get 2x MR). The LHR flights are in KLM aircraft in Y with MF codeshare numbers. Just like your flights. I contacted MF about the KLM flights being in Y and was told that’s the way it is when they sell tickets on other airline’s aircraft. Not fussed being in Y for very short flights as the seat is the same and I still have lounge access and SkyPriority. But the lower earned FB Miles and XP is annoying.

Flown MF B789 J class before and product was good so happy to take it again and the price was low.