I've been noticing this and wondering how do I prevent it.

Regardless of what materials I use, custom or built in, if I hold down the right mouse button for paint too long for some reason the terrain textures block out at the edges. This only happens using an image to paint but painting designs in 12km of terrain isn't really doable else.

Here's the video:

pls attach the 2 ground textures you are using and the mask image pattern for the brush