Landscape layer blend - painting artifacts

I’m getting visible hexagonal / circular artifacts when painting landscape layers.

I’m also sharing a screenshot of my landscape material for reference. I’m a bit of a newbie with landscape materials, but from what I can tell this doesn’t seem to be an issue with the material functions themselves — when I fill the entire landscape with a single material, everything looks fine.

The artifacts only appear when I start blending / painting multiple materials. Additionally, the grass layer doesn’t seem to work when painting, even though the grass material itself previews correctly in the Content Browser.

Any insight or suggested workflows would be greatly appreciated.