Check to make sure your ‘Engine Scalability Settings’ for ‘Textures’ is set to ‘Epic’ and the ‘Material Quality Level’ is set to ‘High.’
Also, your Landscape Material set up is missing some important instructions. When painting on your Landscape, you will want ‘Landscape Layer Coords’ and not Texture Coords node. Also within the ‘Layer Blend’ node, you have the option to change the blend mode between ‘Weight Blend’, ‘Height Blend’, and ‘Alpha Blend’.
These, as well as the ‘Landscape Layer Info’ settings you apply for each painted layer will change how your layers appear to blend between one another. Take a look at the images below from my own test project as to how I have my material set up to be able to paint on my landscape.
Landscape Material Set Up
I would also take a look at the Landscape Technical Guide documentation so you can get a better idea of how to troubleshoot your own landscape issues and the correct set up to use in certain cases.
Landscape Technical Guide
Let me know if you have further questions.
Cheers,