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.