I may have solved the problem.
I created a 100x100 grid mesh in Blender, and then selected the middle vertex, applied the proportional editing and then typed gz0.01 to grab the middle vertex and move it upwards by 0.01 units.
This created a slightly bent upwards grid mesh where the middle is slightly higher than the edges of the mesh, so now the mesh is no longer completely flat.
Here’s the result when I drag the mesh into the scene and enable Nanite on the mesh:
And with the triangles Nanite visualisation:
And in the mesh editor with Nanite enabled:
It seems that Nanite doesn’t like completely flat surfaces with lots of subdivisions.
You have to either have just a few subdivisions, or alternatively make the mesh very slightly bent and therefore not flat.