Reproduction Steps:
1. Open project
2. Run project in editor
3. Notice that Plane has some textures
4. Build to iOS device
5. Notice that textures on panel are not showing

Expected behaviour: Plane looks the same on device as in editor.
Actual result: On device Plane has no texture.

Reproduced on Graphical API GLES3 and Metal

Reproduced on versions: Unity 5.4.3p3, 5.5.0p1, 5.6.0b1
Did not reproduce on 5.3.7p2 (did not compile)
Reproduced on devices: iPad Mini 2 iOS 10.1.1, Samsung Galaxy S 6 edge[SM-G925F], Mali-T760MP8 , OS version 6.0.1

RESOLUTION:
The actual issue here is that the user is calling Texture2DArray.Apply() after copying data to it via CopyTexture(). This can't work -- CopyTexture() operates exclusively on data on GPU side, whereas Apply transfers data from CPU-side copy to GPU-side copy and overwrites it. Omitting the Apply() call results in expected behavior. The documentation has been fixed to reflect this requirement.