Left and right scenarios identical apart from nanite being enabled on the right landscape.
Opacity masks work fine however for nanite meshes, just not landscapes.
Interestingly when the nanite landscape is selected, the cutout lines are shown - but maybe I’ve missed some setting to cause ue5 god to press out the pieces.
Noticed in UE5.6, but also tried latest UE5_main build with no success. Have rebuilt the nanite data.
(Ignore the one-minus)