Since some of the newest updates, terrain painter only changes the physics of the terrain, and dont touches texture. To change texture of terrain, you need to edit .png file or something, with is hard.It was more intelligent before; maybe reset the changes, so terrain painter will both change physics and texture of edited terrain.
It's not really changed. It just depends on the settings. The old textures used base textures in the same way, they just didn't have the additional features of pbr. Either way, progress is forward, not backwards! You can still use pbr textures how you want, but you need to understand how beamng textures work first, and they're complicated. Just when I think I have it, I find something new.
