arbitrary orientation problem with Readpixels and render texture when I cut texture to splinters

In my game I make a screenshot of a cube with text on it, replace the cube by 64 Splinters and "explode them". Everything works fine on my Mac AND on my iPhone, but not on my iPad, even when I run the exact same app on both iOS devices.

The error appears when I move my "TextureCam" on the Cube, create a new RenderTexture, and use ReadPixels to get the 16 small areas to create the 16 textures, for every front panel Texture one.

The strange thing is that the lines get mixed up, and the reason must be, that there is a difference NOT in the orientation of the original cube and the TextureCam (and thus the RenderTexture), BUT in the orientation of the ReadPixels function. Once it reads from bottom to top (as it is documented), but sometimes it reads from top to bottom.

My problem is that I can't find a way to "reorientate" ReadPixels OR to at least to know which orientation it has right now.

Obviously, I tried to fix the orientation in player setting and on the device. More information on how I could influence the orientation of ReadPixels would be greatly appreciated!