Making Metahuman use UE5 MannyQuinn Skeleton

Oh I tried, the Assign Skeleton thing I mentioned was slightly off topic.

Just to be sure I tried it a second time now, but still it forces the extra MH bones to the UE5 Mannequin (it says so in the Output log, and I checked the skeleton and indeed they are listed in the outliner.
Original UE5 mannequin:

After replacing MH (notice the extra finger bones):