There was an old A6 shader named terraintex3.fx. The terrain shader in A7 and A8 has the name terraintex.fx (without the "3").

Hmm.. On A7 and A8, when I open the mtlFX.c, at line 494 there's a mtl_terraintex3 material and fx_terraintex3 action. That's what I use. It worked fine on A7, but when I shifted my game to A8 and compiled the code, the shader doesn't appear already. It's rly weird, cuz I didn't change anything - the action is attached to the terrain, the skins are in the folder, the terrain skin_1 is the RGB map...

The original A6 terraintex3 shader can't work anymore because model 1 pixel shaders were abandoned by Microsoft long ago, I believe in 2007. So, first check if you're really using the compatibility version that was included in A7, and not still the original A6 version. All A7 shaders used model 2, so they should work unchanged in A8.

Apart from that, I don't know what the problem might be - you can contact Support and send them the project for looking into. The best solution, of course, it to switch to the current terrain shader.

Yeah, seems like my main script has been compiling with the old mtlFx shaders...
It's a little bit silly, but that's the bad thing of having old versions of the engine on the same OS...
When I copied the mtlFx.c from the A8 folder into my project's folder it worked fine.

Thanks, JCL for help.