Steps to reproduce:
1. Open attached “GetMaterial.zip” Unity project
2. Open “Main” scene
3. Select "Text" object (child of Canvas) in Hierarchy
4. Enter Play mode
5. Observe Material Temp value in the attached script component in Inspector

Expected results:
GetComponent<CanvasRenderer>().GetMaterial(1); returns the material at index 1 and Material Temp value is ‘Mat2'
Actual results:
GetComponent<CanvasRenderer>().GetMaterial(1); returns the material at index 0 and Material Temp value is ‘Mat1'

Reproduced in: 2018.1.0a7, 2017.3.0f1, 2017.2.0p4, 2017.1.2p3, 5.6.4p3, 5.5.5p1, 5.5.0a6, 5.4.6f3
Fixed in: 2018.2.0a1