When the textured wall top is under the heightfield top, the texture of the heightfield wall appears.
Only way to fix:
-Change the wall texture of the slope.
-Savemap to test
But it will not work for your case, I will try to explain how to fix it especially for your case in English.
The solid wall (F wall) must be non-solid anymore. Press "G" (to make in non-solid) hide the low part of the non solid wall under the high part, then texture the hidden part of the wall with the one you want at the texture bug.
Tip: use the skymap texture on the floor, like this you can't see there is such a fucked wall with a hidden part on the radar.
Only way to fix:
-Change the wall texture of the slope.
-Savemap to test
But it will not work for your case, I will try to explain how to fix it especially for your case in English.
The solid wall (F wall) must be non-solid anymore. Press "G" (to make in non-solid) hide the low part of the non solid wall under the high part, then texture the hidden part of the wall with the one you want at the texture bug.
Tip: use the skymap texture on the floor, like this you can't see there is such a fucked wall with a hidden part on the radar.