Empty squares and floating when spawning grass with imported layer

[anonymous_user_c7b439e8]

Does anyone know what might cause these artifacts?

I am using a massive landscape (100kmx100km) without World Composition taken from a 2160x2160 pixel heightmap. The grass is an imported png (grayscale) layer of the same size. In the material the layer “grass” is simply plugged into a LandscapeGrassOutput node from a LandscapeLayerSample called “grass”.

I already made sure these squares aren’t on the original image.

Also, perhaps related to this: the grass floats in the air on some gradients:

Thanks!