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.
My next flight in May to MNL has the outbound DUB-AMS sold by MF as KLM coded and therefore Business. The return AMS-DUB is MF coded only
But there is not enough time for the lounge anyway so a pint of Heineken and an Exit row seat does it as well.
BTW: When you get your boarding pass in Xiamen printed for the shorthaul Economy class within Europe they print a "Sky Priority" line on it if your inbound to AMS is in buisness. Hold on to that boarding pass as a re-print in AMS (at least the one from the vending machines) will not show this.