It is for this texture bug that … after a very long time … I decided to go back to DX11.
I avoided DX11 because it had another, even worse, landscape bug, with completely messed up “textures” (e.g. water tiles in the forrest) … but so far, after over a week of DX11 … I can say that I have not see the DX11 bug anymore (on my AMD RDNA3 GPU).
So I am sticking with DX11 … until this ground texture issue is fixed.