How to reproduce:
1. Download attached project file and open scene 'scene'
2. Select Gameobject 'Main Camera (1)'
3. Make sure the scene view is docked next to the game view to compare.
4. Enter Play mode

Actual result: Camera preview in the scene view, the shine wraps over the object perfectly. In the game view, which shows a RenderTexture with a view of the main camera, the shine does not wrap

Reproduced with: 2017.2.2f1, 2017.3.1p2, 2018.1.0b9, 2018.2.0a1

-----------------------------

By design: The RenderTexture is created using 16 as depth. Only a depth of 24 will enable stencil features like SpriteMask (https://docs.unity3d.com/ScriptReference/RenderTexture-ctor.html).