Which version of Editor are you using?
The Nanite setting there is a setting that allows for ‘Preserve Blended Space’ or something similar, it is basically for polygons supporting Materials that have Alpha.
When/If I use Nanite on Flora (Trees/Plants), I only apply the nanites to the Trunk, not the leaf sections, they are mostly scant polygons and nanite is 100% overkill.
Are you using Distant Meshes?
Maybe turn off Mipmaps.