I've just had my first real try with Unity here but I think I managed to figure out what causes this.
What I did to get it to display when building for iOS was setting Import Settings Format of the texture to Truecolor. After this I went to build settings and added the current, hit Build and it was displaying again.
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If the texture is not square, iOS does not allow compression of it, so if you want to use compression you need a square atlas texture.
I hope this help.
